Death in the Cosmos

 

Death, is it the end of existence?

A large star will end its life in a supernova. A spectacular explosion that allows the star to outshine the galaxy it resides in. This explosion will seed the cosmos with elements that were made within the star as it aged, along with elements (like gold and platinum) that were directly produced during the supernova.

Supernova in the galaxy NGC1365. Image courtesy of Martin Pugh (http://www.martinpughastrophotography.id.au/)

Our Sun will not explode; it does not have enough mass to generate a supernova. But, it will eventually become a red giant and engulf the Earth, as it begins to burn helium after exhausting the hydrogen at its core. As the star consumes its helium it will go through a series of oscillations, shrinking and expanding. This process will blow off layers of material from the star that will form a nebula, called a planetary nebula, which will mark the location of our star as it shrinks down to a small white dwarf star and slowly cools over the eons.

The Ring Nebula, a planetary nebula 2000 light-years away. Image courtesy of NASA.

In both cases, material from the star is returned to the cosmos that can become the seed material for a new star and planetary system. So, the death of a star is really the beginning of a new generation.

Humans, every plant, animal, rock and drop of water on this planet were all formed from the elements cast into the Universe by stars that have passed out of existence. When we pass from the world of the living and are interred on this Earth, we will return our borrowed elements to Mother Earth. And, when the Sun goes through its final stages of life and our planet is consumed by the bloated star, everything on it will be returned to the cosmos and become the raw materials for a new solar system and maybe someday, a new form of life. As Carl Sagan said, we are all star stuff.

But, what of that collection of electrical impulses makes each of us unique, that makes us human—the soul, if you will? What happens to that entity?

Those with religious beliefs will say that the soul moves on to heaven or hell or some other after-life, depending on one’s conduct on Earth. Scientists may try to measure the change in electromagnetic energy a person has after death. But, the former provides no proof that we transcend to a higher plane of existence, while the latter only quantifies the energy we possess and doesn’t reveal the unique life-force it contains. The bottom line is that we just don’t know. That spark which makes us – us, is surely contained within the bounds of the cosmos. But, what it is, how it works, where it comes from and where it goes is a mystery.

Just as the star lives on by casting its elements throughout the cosmos, so does a person live on through the people they encounter in day-to-day life, from family to friends to co-workers to strangers. We all can carry some part of the essence of that person into the future, and we will pass it on to our friends, family and acquaintances through the stories we tell and the actions we take.

This cosmos we live in is so vast, with so many unknowns. We have many questions to answer, and many more to ask.

This post is dedicated to the memory of my father, Edwin Allen Davison Sr. (June 30, 1925 – September 21, 2013). His spark has been returned to the cosmos. I will miss our discussions of the wonders of our Universe…

Hubble Extreme Deep Field. Image courtesy of NASA

Till next time,

RC Davison

 

Neutron Stars, General Relativity and Elephants

The discovery of an unusually massive neutron star with a white dwarf companion was revealed in a paper published by John Antoniadis, a PhD student at the Max Planck Institute for Radio Astronomy and others on the international team this past April. Using radio telescopes from observatories around the planet to identify and study the neutron star, and the European Southern Observatory’s (ESO) Very Large Telescope (VLT) with its FORS2 spectrograph located at the Cerro Paranal observatory in Chili, to study the white dwarf star, the astronomers have discovered the most massive neutron star found to date. Labeled as PSR J0348+0432, the neutron star weighs in at twice the mass of the Sun.

White dwarf star orbiting a pulsar, a neutron star beaming radio frequency energy, generating gravity waves as they revolve about a common center. Image courtesy of ESO

So what? One might ask.

Well, what’s remarkable is that this mass exits in a sphere only 12.4 miles (20 km) in diameter. This means that the density of the material inside this defunct star is on the order of 1 billion tons per cubic centimeter—the size of a sugar cube! The force of gravity on the surface of the star is 300 billion times stronger than what we experience here on Earth.

This super dense ball is rotating 25 times per second and has a white dwarf companion star with a mass 0.17 that of our Sun and a diameter of 56,000 miles (90,000 km) orbiting it every 2.5 hours. This neutron star is also a pulsar. It sends a highly directional beam of radio frequency energy out into the cosmos and provided the pulsating beacon that we detected to locate this unique stellar system.

This super massive star along with it’s companion provides a wonderful natural laboratory for Earth based astronomers to study Albert Einstein’s General Theory of Relativity, which describes how space is curved by mass and energy and we observe in part as gravity. Studying this high intensity gravitational system may help us better understand gravity waves, predicted by Einstein’s theory, and explore the realm where general relativity and quantum mechanics may meet.

The team of astronomers have already measured a reduction in the orbital period of 8 millionths of a second per year due to energy being radiated from the system by gravity waves, as predicted by general relativity. Although gravity waves have been inferred by this and other binary systems, they have yet to be detected by the Laser Interferometer Gravitational Wave Observatories—LIGO—facilities on Earth.

But, wait! A sugar cube size piece of neutron star stuff that weighs 1 billion tons? How do you wrap your head around that piece of information? How do you compare that to everything you touch in your day-to-day routine? Let’s see what a billion tons of stuff might look like.

A good, massive object that most people have a concept of might be the African bull elephant.

Weighing in at about six tons on average, ten feet high by twenty feet long and eight feet wide, we would need only 167 million bull elephants to equal one cubic centimeter of neutron star material. That’s a lot of elephants!

To get a better perspective on how large this number of elephants is, consider if you packed these pachyderms side by side, front to back, you would cover an area of 26.7 billion square feet. (Whoops! We’re back to billions again. Better to convert that to square miles/kilometers.) That’s 956.5 square miles (2477 sq km); equivalent to a square with sides 30.93 miles (49.8 km) long. You could comfortably park them all in the tiny country of Luxemburg, which has an area of 998 square miles (2586 sq km), with a little room to spare.

How about something bigger, even more iconic, like the Empire State Building (ESB). Standing 1,454 ft (443.2 m) high, it has an estimated weight of 365,000 tons. We would need only 2740 ESBs to offset a balance with 1 sugar cube-size piece of neutron star stuff on it. That’s at least a number we can begin to have an intuitive sense for.

So how much area would 2740 ESBs cover? With a foot print of 79,288 ft2 (7240 m2 ) or .003 square miles (.007 square km), our collection of buildings would cover 7.8 square miles (20.3 sq km) – about 1/3 of the island of Manhattan, which has an area of 22.96 square miles (59.5 sq km). It’s a bit hard to imagine a third of Manhattan covered in Empire State Buildings. But, we can reduce the number and get a better handle on a billion tons.

Let’s take our Empire State Building and make it completely out of gold, all 37 million cubic feet (1.04 million cubic meters) of it. With gold weighing 1204 pounds per cubic foot, the solid gold building would weigh 44.5 billion pounds or 22.3 million tons. Now all we would need is 45 of these precious metal buildings to reach 1 billion tons.

This gilded collection would cover about 23 city blocks or an area from where the ESB is now to Times Square, assuming two buildings per block. Try to imagine this the next time you fly to New York City: the core of downtown Manhattan populated with 45 gleaming, solid gold Empire State Buildings and all that is equivalent to 1 cubic centimeter—one sugar cube-size of neutron star stuff.

Hopefully this helped you get a little better feel for what a billion tons might be. It’s helpful to do these simple calculations and comparisons and try to put into perspective or get a better grasp on some of the enormous numbers that come out of the study of this amazing Universe we live in.

When considering the cosmos and all the numbers we produce to describe it, I cannot help but feel that all we hold dear on this tiny blue planet, floating through the vastness of space, is insignificant when compared to what we are immersed in. Yet, we are sentient beings, and curious about the Universe we live in and that makes us very significant, because for all we know now, we are the only creatures in this entire cosmos that are looking up and asking these big questions.

Till next time,

RC Davison

Link to the research paper: “A Massive Pulsar in a Compact Relativistic Orbit”, by John Antoniadis et al.

 

 

Asteroid 2012 DA14, Tunguska Impact, Meteor Crater, and the Russian Meteor of 2013

(Post updated 2/19/2013 with latest assessment on asteroid from ESA.)

Wow! Two wake up calls for the planet Earth in one day! Maybe it’s about time that the people of planet Earth realize that they are inside the pinball machine that makes up our Solar System. Sooner or later that ball is going to hit us head on. Today we were lucky – twice!

Russian meteor, February 15, 2013

The spectacular meteor that streaked across Russia’s sky Friday morning has been estimated to be about 56 feet (17 meters) across, weighing in at more than 7000 metric tons and moving at speed around 40,000 mph (64, 373 km/h). It exploded about 9-12 miles (15-20 km) above the surface of the Earth with an equivalent of 500 kilotons of TNT—30 times the energy of the Hiroshima atomic bomb.  The consequent shockwave shattered windows and damage buildings in and around the Russian city, Chelyabinsk, resulting in over 1000 injuries.

This meteor was not related to the flyby later in the day of asteroid 2012 DA14. This asteroid skimmed by the Earth at a distance a little over 17,000 miles (27,400 km). Friday, February 15, 2013 could have turned out a lot different if either of these cosmic messengers had a slight change in course, which in the case of the Russian asteroid, could have detonated lower and over a more populated area or for 2012 DA14, a direct hit instead of a near miss.

We have two good examples of the consequences of an asteroid the size of 2012 DA14 (150 feet, 45 meters, ~130,000 metric tons) hitting the Earth in the Tunguska explosion of 1908 in Siberia (120 feet, 37 meters, ~100,000 metric tons) and the nickel-iron meteor (150 feet, 50 meters, ~270,000 metric tons) responsible for Meteor Crater in Arizona.

Map of Tunguska Impact (Sullivan 1979 and Kridec 1966.)

The Tunguska explosion occurred in the air above Siberia at a height of about 28,000 feet

(8500 meters) and generated the equivalent energy of about 1000 Hiroshima atomic bombs. The result was over 800 square miles of forest destroyed and a shock waves that

were recorded as far as western Europe and registered a magnitude 5 earthquake. As of today, no crater has been found to mark an impact of the remnants of the asteroid, leading some to think it might have been piece of a comet that entered Earth’s atmosphere that day, which is made mostly of ice.

Meteor Crater (AKA Barringer Crater) Arizona – Wikimedia Commons

Contrasting Tunguska is the nickel-iron meteor that did leave a crater in what is now Arizona. About 50,000 years ago this meteor entered the atmosphere at a speed of about 27,000 mph (43,000 km/hr) and fragmented to some degree due to the stresses associated with entry into the atmosphere, but the bulk of it hit the Earth creating a crater that is 4000 feet (1200 meters) in diameter and 570 feet (174 meters) deep. The explosive energy released from the impact has been estimated to be as high as 200 times that of the bomb dropped on Hiroshima.

Impact effects at Meteor Crater – Image courtesy of the Space Imagery Center and/or David A. Kring

We see two very different effects from two similarly sized asteroids.  But, it is the different composition that makes the difference.  The high density nickel-iron meteor survives the descent to the surface, while the less dense, ice-rich meteor fragments due to the high stresses experienced in its passage through thicker layers of the atmosphere. The temperatures experienced by these fragments can reach 45,000 °F (25,000 °C) causing the massive fireball and resulting shockwave and destruction.

We don’t, by any stretch of the imagination have knowledge of every asteroid in the Solar System that poses a potential threat to Earth.  The more we look the more we see, and with regard to near Earth asteroids (NEA), the sooner we find them the better.  It is possibly the only natural disaster we may be able to avert, given enough time.

Till next time,

RC Davison

References:

Russian asteroid impact ESA update and assessment

The Tunguska Impact – 100 Years Later

Damage by Impact — the Case at Meteor Crater, Arizona

Barringer Meteor Crater and Its Environmental Effects

 

Planet Found in the Alpha Centauri System – Could Pandora Be Discovered Soon?

Artist's illustration of the Alpha Centauri System. Credit: ESO/L. Calçada/Nick Risinger (skysurvey.org)

Reminiscent of the movie AVATAR, a planet has been discovered in the nearest star system to our Sun, Alpha Centauri. This is a trinary system consisting of three stars: Alpha Centauri A, B, and C. Alpha Centauri A is the same type of star as our Sun but slightly larger while its companion, Alpha Centauri B is slightly smaller and cooler. Alpha Centauri C is a red dwarf star also known as Proxima Centauri and is the closest star to our solar system at a distance of 4.22 lightyears. Alpha Centauri A and B orbit each other at a distance of about 23 AU (Astronomical Unit: 93 million miles/150 million kilometers) or about the distance between the Sun and Uranus.

This newly discovered planet is no Polyphemus, the gas giant in the movie that the moon Pandora orbited. The planetary system was in orbit around the star Alpha Centauri A. This planet (designated Alpha Centauri B b) is in orbit about Alpha Centauri B and has an orbital period or year of 3.236 days. It’s mass (minimum mass) is 1.13 times that of Earth and it orbits its star at a distance of about six million kilometers, 3.6 million miles.

The simple facts about this planet belies the huge effort that was put forth to push the envelope of the technology and analysis techniques to find the planet.  This information was gleaned out of data collected from over of four years of observations using the HARPS spectrograph at the ESO LaSilla Observatory (See Finding Exoplanets – Part 2: It’s All About the Mass for more information on the HARPS instrument.) The team of astronomers, lead by Xavier Dumusque (Geneva Observatory, Switzerland and Centro de Astrofisica da Universidade do Porto, Portugal), lead author of the paper were able to improve on the sensitivity of the HARPS instrument by taking into account:

  • The radial motion of the Alpha Centauri star system relative to Earth
  • The stellar oscillation modes for Alpha Centauri B, akin to seismic vibrations
  • The granulation of the star’s surface (the convective zones of rising hot plasma and sinking cooler plasma on the surface, which contribute noise to the measured radial-velocity of the star)

    Image of the granulation of the Sun's surface. Image courtesy of ESA

  • The rotational contribution of the star (as the star rotates, the side moving toward us will be blue shifted while the side rotating away from us will be red shifted)
  • Spots on the surface that are brighter or darker than the mean
  • Magnetic cycle activity
  • Light from Alpha Centauri A contaminating the spectrum of the B star
  • Instrument noise.

After extensive data reduction and analysis, the team determined that the star was wobbling at a velocity of 51 cm/sec (20 inches/sec) due to the planet’s motion. This is about 1.8 km/hr or 1.1 mile per hour!

Although the planet discovered is too close to its parent star to be habitable, at least with life as we know it, the analysis techniques developed to pull the presence of the planet out of the noise can be used to identify planets with a minimum mass of 4 times Earth’s mass in the habitable zone of a star. This opens up a new category of planets that can be searched for.  Note that this is the first planet found in the Alpha Centauri, it may not be the last. It may only be a matter of time before a planet (or moon) like Pandora from AVATAR is found in a star system in the Milky Way.

Till next time,

RC Davison

References:
Planet Found in Nearest Star System to Earth: http://www.eso.org/public/news/eso1241/
Paper: http://www.eso.org/public/archives/releases/sciencepapers/eso1241/eso1241a.pdf

Lithium, Stars and Planets

On Earth, the element lithium has certain medicinal properties when applied to conditions like depression and bipolar disorder, and it is extensively used in the battery technology powering most of our portable electronics. In stars, the amount of lithium present is an indicator of the age of a star.

The older the star is, the lower the concentration of lithium measured in the photosphere – the part of the star that we can see. Typically as a star ages, lithium is moved through convective motion deeper into the star where the temperatures are higher and the element is consumed. When astronomers find a star that shows a higher than normal lithium content for its age, eyebrows get raised and heads get scratched.

After the big bang, the Universe (by mass) was about 75% hydrogen, 25% helium and extremely small trace amounts of lithium, all the other elements we have today have been synthesized in stars as they move through their normal life cycle.  Elements heavier than iron are produced when the more massive stars explode as supernova.  The first stars that formed after the big bang (called Population III stars) reflected the amounts of hydrogen, helium and lithium originally present.  Second generation stars (Population II) contained higher levels of the elements heavier than lithium thanks to the first generation enriching the cosmos, but these are considered “metal poor” when compared to Population I stars, like our Sun.  (Astronomers consider any elements heavier than helium to be metals.)

The planets that form around a star contain the primordial elements of the big bang, along with whatever new elements have been seeded in the protoplanetary dust cloud from novae and supernovae. Lithium is preserved in the relatively cold planets as they condense and solidify. If a planet containing lithium is pulled into its parent star, it will disintegrate, spreading its contents though out the star’s atmosphere. This mechanism can explain how a star can have a higher than normal lithium content for its age.  But, this process is transitory.  Eventually, the lithium will be processed by the star.

There have been two recent observations of stars that show higher than normal amounts of lithium:

One is associated with a red giant star (BD+48 740) that is suspected to have at least one planet orbiting it in a highly eccentric orbit. Dr. Alex Wolszczan, professor of Astronomy and Astrophysics at Penn State University, has led the team which discovered this youthful red giant. Evidence indicates that the star has a massive planet in a very elliptical orbit, which is unusual but can be attributed to gravitational interactions between planets in the solar system. This interaction may have contributed to another planet moving too close to the parent star and being engulfed as the red giant swells with age, giving rise to the higher than normal lithium content.

Red Giant engulfs one of its planets. (Image courtesy of NASA)

The other observation is of a star (#37934) in the globular cluster NGC 6121, also known as Messier 4 or M4.  The ESO (European Southern Observatory) has released an image of M4 and discusses the surprising discovery.

Globular cluster M4, NGC 6121 (Image courtesy of the European Southern Observatory)

This star peculiar in that it is exhibiting a much higher than normal level of lithium for the ancient stars (Population II) that typically make up globular clusters. In the paper presented on this observation the authors present two scenarios that may explain this star’s unusual concentration of lithium.  The first is that the star formed with a higher than normal amount of the element  – i.e. it was polluted by its environment.  The other thought is that the star, for some unknown reason, hasn’t processed the lithium like the rest of the stars in the cluster.  Both ideas are up for debate as there isn’t enough evidence to prove either one correct.

But, could this star have sacrificed one of its planets for a brief period of youthful lithium enrichment like BD+48 740? (This assumes that it has or had planets orbiting it.)

Star in M4 exhibiting higher than normal lithium levels. (Image courtesy of the European Southern Observatory)

Perhaps the discovery by Dr. Wolszczan and his team shows a stellar process that is more common than thought.  If one considers the high number of planets being discovered by Kepler, which is leading astronomers to predict even greater number of stars with orbiting planets, this idea may be even more plausible.

Another case of the cosmos leading us down a rabbit hole just like Alice in Wonderland – the more we look, the more we see and the more questions we raise.  The Universe just gets curiouser and curiouser!

Link to published papers:

BD+48 740 – Li overabundant giant star with a planet. A case of recent engulfment?

Lithium and sodium in the globular cluster M4. Detection of a Li-rich dwarf star: preservation or pollution?

Till next time,

RC Davison

Galaxies in Collision

In the vastness of the cosmos it seems amazing that objects run into each other, but they do. The pervasiveness of gravity has dominated and shaped the Universe as we see it today, from simple planets and solar systems to vast galactic clusters containing thousands of galaxies bound together. Galaxies collide, and galactic collisions create some of the most beautiful structures we’ve seen in our search of the cosmos.

Here we have The Mice:

Two galaxies colliding, known as The Mice - NGC 4676. Image Courtesy of NASA/Hubble Space Telescope

The Exclamation Point:

Arp 302 - Two galaxies about to collide - Image courtesy X-ray NASA/CXC/IfA/D.Sanders et al; Optical NASA/STScI/NRAO/A.Evans et al

Sometimes what appears to be a collision about to happen is really a case of one’s perspective, as can be seen in this image from the Hubble Space Telescope of NGC 3314.

Colliding galaxies? Not really. Image courtesy of NASA/Hubble Space Telescope

The galaxy that we see almost face-on – NGC 3314a is in the foreground and is tens of millions of light years from the background galaxy NGC 3314b. These two galaxies will not become another statistic in the annals of galactic collisions. But, the same can not be said for our own Milky Way galaxy and the Andromeda galaxy (M31).

In the next four million years or so, these two galaxies will begin to become one through a graceful pas de deux that will take millions of years and result in what theory predicts will be a large elliptical galaxy. This information, along with some amazing simulations and illustrations can be found at the Hubble Space Telescope’s site.

Here’s a graphic illustrating the collision showing the paths of the two galaxies along with another galaxy in our Local Group, Triangulum (M33):

Illustration of the Milky Way and Andromeda galaxies ultimate demise. - Image courtesy of NASA/Hubble Space Telescope

In this artist’s conception, the collision is seen from the perspective of an observer on Earth.

Illustration of the Andromeda galaxy's approach - Image courtesy of NASA/Hubble Space Telescope

The last few frames shows how Andromeda dominates the night sky and effectively blocks our view of that portion of the Universe. Future astronomers will not be able to appreciate the night sky as we are able to today.  But, who knows if humans will still be observing the Universe by the time this event takes place.

Looking at these images I can’t help but wonder about the alien astronomers living in the Triangulum galaxy. What a spectacular view they have of this doomed pair of galaxies. I wonder if they have mapped out the motions of these island universes (as they were once known) and understand that they will eventually collide. And, even more mind-boggling: Are they looking at us and wondering if someone is looking back?

Till next time,

RC Davison

 

Andromeda – Beyond the Blue

Hopefully we are all aware of the fact that ultraviolet rays from the Sun are bad for our skin.  The reason they are hazardous is because of the high energy that that they possess, which allows them to penetrate our skin and damage the cells internally. UV is a small part of our Sun’s emissions but UV radiation is a major component of the energy emitted by very hot, massive stars, as can be seen in the image below from NASA’s Galex (Galaxy Evolution Explorer) satellite.

Andromeda Galaxy in UV - Image courtesy of NASA

These stars that line the arms of the Andromeda galaxy are the result of dust and gas that form the structure of the arms and consequently, the birthing place of new stars.  Blue giants have very high surface temperatures ranging from 10,000 to more than 40,000 degrees Kelvin.  The more massive the star, the hotter it is and the more it will radiate in the ultraviolet.  But, running hot and massive comes with a cost.  These blue giants will burn out in supernovae in a few tens of million of years. (A very short time – astronomically speaking!)  Compare this with our Sun, which has a surface temperature of about 6000 degrees Kelvin and will be around for at least 10 billion years.

Below you can see Andromeda in a Hubble image in the optical spectrum fading to the ultraviolet image from Galex.  It’s easy to see that these high-powered stars reside in the dusty arms of the galaxy.  In a few million years the Milky Way Galaxy will have a ring-side seat to view these blue giants as they spectacularly end their lives!

Andromeda from Hubble in visual and UV from Galex

For more on the Andromeda galaxy take a look at an earlier post to see Andromeda in a different light: The Many Faces of Andromeda.

Till next time,

RC Davison

Dwarf Galaxy NGC 2366 and Beyond!

A smudge in the night sky can contain many wonders!

Ground-based, wide-field view of NGC 2366 from Digital Sky Survey 2 - Image courtesy of NASA

The blue streak in the above image is the dwarf galaxy NGC 2366.  It is about 10 million lightyears distant and located in the constellation Camelopardalis (the Giraffe), which is visible in the northern hemisphere.  Barely visible to the bottom right of the blue smudge is a bright spot, which is an active star-forming nebula, NGC 2363 contained within the dwarf galaxy.  In the image below you can see the nebula shining from the light of the hot blue stars that are forming in the upper right part of the galaxy.

Hubble view of NGC 2366 - Image courtesy of the NASA/ESA Hubble Space Telescope

Zooming in on the nebula in another Hubble image below, one can see the collection of bright stars embedded in the nebula.  Of particular note is the very bright star that appears at the tip of the “hook” of the nebula.  This massive star is known as a Luminous Blue Variable (LBV), which is about 30 to 60 times as massive as the Sun.  This is a very rare type of variable and very unstable.  The image captured the star during an erupting phase.  Another, more famous star of this type is the giant, Eta Carinae, which is anticipated to turn into a supernova in the near future (astronomically speaking).

NGC 2363 - Star forming nebula in the dwarf galaxy NGC 2366 - Image courtesy of NASA

When you look closely at the image of NGC 2366 you will see many “nebulous” regions within it.  They are actually very distant galaxies that are visible through the veil of the dwarf galaxy.  I’ve highlighted some of the more prominent galaxies that can be found in the image below.

Galaxies beyond NGC 2366 - Base Image courtesy of NASA/ESA Hubble Space Telescope

Here is a composite of some of the major galaxies hiding behind this dwarf galaxy.

Galaxies behind the dwarf galaxy NGC 2366

NGC 2366 is just another example of what wonders are hidden within the smallest parts of the night sky and what amazing things are awaiting discovery.

Till next time,

RC Davison

Resource: Hubble Space Telescope NASA/ESA Hubble Observes a Dwarf Galaxy with a Bright Nebula

Finding Exoplanets – Part 2: It’s All About the Mass

In the first blog on finding exoplanets I discussed the transit technique that is used to find planets by the light they block as they pass in between their star and the Earth. That gives information on the size of the planet, its orbit as well as information on the atmosphere of planet if it has one. But, there is a critical piece of information that is not supplied by the transit technique and that is the mass of the planet. Finding the mass of the exoplanet is done by measuring its radial velocity – its velocity as it moves toward and away from us as the planet or planets pull on it while orbiting the star.

Theoretically, one could measure a star’s motion relative to stars more distant as a planet tugs it one way or another, but in practice it is extremely difficult to measure this tiny displacement. This is the branch of astronomy known as astrometry. This technique has has not had much success due to limitations in the optics used in the telescopes and atmospheric turbulence. The more distant the star of interest is, the more imprecise the measurements become.

The technique that has been very successful utilizes the Doppler shift or the change in frequency of star’s light as it is pulled back and forth by its planets. Just like the sound of a train whistle increases in frequency or pitch as it moves toward you, and decreases as it moves away, the light from the star increases in frequency (gets bluer or is blueshifted) when the star is pulled toward us and decreases in frequency (gets redder or is redshifted) as it is pulled away from us.

To use this technique the light from the star is broken into its constituent parts by using a spectrograph and the shift of the spectrum can be measured with great precision as the star moves, allowing astronomers to calculate the minimum mass of the planet(s) that orbit the star.

The shift of a star's spectrum as it is pulled by an orbiting planet

(Credit: University Cooperation for Atmospheric Research)

In the image above, one can see the star moving around a center of mass determined by the mass of the star and the planets orbiting it. The spectrum shows black lines which are created by elements in the star’s atmosphere that characteristically absorb electromagnetic energy—light—at different frequencies. These lines are used to determine the composition of the star’s atmosphere, but it is the displacement of these lines that astronomers measure to determine the radial velocity and then the minimum mass of the planet. The last image in the graphic shows the absorption lines as they shift toward the red and blue ends of the spectrum.

Note the emphasis on the minimum mass.  This is the mass calculated by the component of the star’s velocity that is directly toward/away from us — its radial velocity.  If the orbital plane of the planet is not directly in our line of sight (i.e. its orbit is inclined at some angle) then we are not seeing the total velocity of the star due to the planet, only a component of it (Remember your vectors!).  If we don’t know the orbital inclination of the exoplanet to its star, then the mass we calculate based on the radial velocity we measure is a minimum mass and could be higher.  If it turns out that the planet is orbiting in a plane directly in our line of sight, then the mass measured would be the actual mass.

One of the most precise spectrographs in use today is the HARPS (High Accuracy Radial velocity Planet Searcher) spectrograph, located at the European Southern Observatory’s (ESO) La Silla facility in Chile. This spectrograph was instrumental in the latest release from ESO “Many Billions of Rocky Planets in the Habitable Zones around Red Dwarfs in the Milky Way.”

HARPS can resolve radial velocities to 1 meter per second. That translates to a star moving toward or away from us at a velocity of 3.6 km/hour, or 2.2 mph—that’s average walking speed for humans! To get an appreciation for these numbers, consider if we were looking at our Sun from a distant star using the HARPS spectrograph.  We would easily be able to detect Jupiter’s influence on the Sun, which provides a radial velocity of 12.7 m/s. Earth, however, would not be detectable because it only disturbs the Sun on the order of .1 m/s.  But, it is not just the size of the planet that determines how much it will affect the radial velocity of the star but also its distance from the star.

Having a value for the mass of an exoplanet allows astronomers to determine the planet’s density and determine its probable composition. Having an educated guess at the planet’s make-up and knowing its distance from its star, and therefore its likely surface temperature, one can then speculate as to the state of water on the planet if present and the likelihood of life as we know it.

More and more evidence is surfacing that keeps increasing the number of potential planets in the average galaxy. The higher the number of planets, the better the odds of finding a habitat that is conducive to life. And, if we extend our theorizing to the number of moons that may orbit these planets, the number of potentiality habitable planets is…well…astronomical!

Extraterrestrial civilization on a moon of a gas giant in a distant solar system. -RC Davison

Till next time,

RC Davison

Finding Exoplanets – Part 1: The Transit Method – Not Quite Point and Click!

With Kepler increasing the total number of potential planets discovered almost daily, (as of 3/10/2012 the total is at 2321 with 61 planets confirmed) it is becoming easier to take this exciting news for granted. There is a lot more going on behind the scenes to these discoveries than just pointing a telescope at the stars and waiting for the planets to go by. Kepler uses a technique called the transit method to identify potential planets, but there are other ways to find planets, which I will discuss in a future post.

The transit method requires that the planets revolve around their stars in a plane that is within our line of sight, so from our perspective the planet blocks the light coming to us from its star. In concept this is pretty straight forward and simple. But, as they say, the devil is in the details.

If you click on the image above you will see a simulation provided by the Kepler site of a planet transiting the star it orbits.  What’s interesting to note is the gradual slope of the curve as the planet first begins to cross the star or ingress and is repeated on the other side when the planet egresses the star.  Also, as the planet transits the face of the star the light curve is not flat but curved due to limb darkening effects.  The curve is not smooth, which indicates the variability in the brightness across the star’s surface.  Note that in most cases the amount of light blocked by the planet is only on the order of 1% – 2%.

Consider this.  As you watch the light dim from the star are you really seeing a planet passing in front of it or are you seeing the star itself dim because it is a variable star or has a large starspot (sunspot) on its surface that just came into view or some other phenomena is affecting the light you are seeing?

Maybe the dimming is due to the fact that this is not one star but two in a binary system – two stars orbiting each other in a plane that lies along our line of sight. What you perceive as dimming due to a planet could really be one star eclipsing the other. You would get the brightest image when they are not eclipsing, the dimmest when one is behind the other. These are known as eclipsing binaries. (Note that Kepler has discovered 2165 eclipsing binary stars as of this blog post)  And, what if it’s a trinary system – three stars orbiting a common center of mass?  Throw in a few planets and try to imagine the light curve for that system!

The dimming has to be periodic and repeatable to increase confidence that there really is something out there orbiting the star and not just an intervening asteroid or comet that happened to pass through your field of view. Kepler requires 3 to 4 events to record a potential exoplanet.  The first planets Kepler discovered had orbital periods of several days, which allowed astronomers to gather a set of data in a very short time.  These planets are very close to their star and are extremely hot.

Kepler has been observing for 3 years now, so it will be finishing up data sets on planets that are orbiting further from their star with orbital periods of a year or so.  The longer Kepler looks, the more planets it will unveil.

Through various techniques like spectroscopic analysis one can determine if the star is part of an eclipsing binary pair. Through other observations one can determine if the star is a variable star and if the dimming doesn’t reappear, then it may have been a starspot or other transient phenomena. If the dimming repeats and you collect a set of light curves that show how much light is blocked by the object you can then begin to determine some interesting properties about this object, like how big it is, what its orbital period is and distance from its parent star. But, these numbers don’t come without some hard work.

It’s easy to get a relative size of the planet to its star by how much light it blocks when the planet transits the face of the star.  But, one needs to take into account a number of physical phenomena that will affect the data that is collected. One of these is limb darkening – which is the effect that the star is not as bright at its edges (limbs) as it is at the center. This is due to the fact that one is not looking as deeply into the star at its edges as at its center. This means that the planet will block a greater percentage of light as it traverses the center of the star than at the edge.

Where the planet crosses the star will affect the shape and size of the light curve. Crossing directly over the equator of the star would produce the broadest, most shallow curve while crossing the star at higher latitudes would produce a narrower, deeper curve. This is a reflection of the tilt of the planet’s orbit relative to Earth. These will affect the calculations of how big the exoplanet is and how large its orbit is and must be taken into account.

Transit Light Curves - Image Courtesy of NASA/JPL

Starspots can also skew the data if they occur as the planet transits the face of the star. This is because they are dark and they will add to the amount of light perceived to be blocked by the planet, giving the impression that the planet is bigger than it really is.

An advantage of the transit method is that it allows astronomers to determine if the planet has an atmosphere. Using a spectrometer it is possible to determine the constituents of the planet’s atmosphere as the star’s light passes through the planet’s atmosphere as the planet passes over the edge of the star at the beginning and end of the transit.

The transit method provides information on how big the planet is, once the size of its parent star is known, which is a challenge in its own right.  Once the planet’s size is determined and coupled with mass data gleaned from another technique—radial velocity measurement—the density of the planet can be calculated. When the density is known, approximations can begin to be made about the composition of the planet. Is it gaseous, rocky, or somewhere in between? This information, along with the knowledge of how close to its star it orbits, which determines the temperature of the planet, will dictate what state water might exist if present.

The science of astronomy is an amazing example of how inventive and ingenious man can be.  We have harvested all we know about our Universe from just the light that comes to us through the vacuum of space.

(Check out part two: “It’s All About the Mass“)

Till next time,

RC Davison