What Does An Astrophysicist Do?

Apologies for the hiatus in posts these last few weeks, life and work have been very busy. Since I've been swamped with so much work, I thought I'd take the time in this post to describe what  an astronomer or astrophysicist does on a daily basis.

When you think of life as an astronomer, the first thing that comes to mind is telescopes and star parties. You imagine the scientists out late at night staring through their telescopes and taking notes about what they see. While this part of the job, astronomers have much more to do. Graduate students and professors in astronomy spend most of their time teaching, doing research and applying for grant money. They teach or assistant teach college courses, and are constantly writing proposals to different organizations asking for money to fund their research. But what does "doing research" actually mean? In astronomy, research can mean one of three things: taking images with a telescope  and analyzing them using a computer (observational astronomy), writing computer programs to simulate interactions between objects in outer space (theoretical astronomy), or building telescopes, cameras, and detectors for astronomers to use (instrumentation). The first two require you to sit at a computer most of the day and  write computer programs to perform certain tasks. Observational astronomers also spend a lot of time applying for observation time on both space and ground based telescopes. If their proposals are accepted, they receive images from the telescope that they can then analyze to understand the physics and properties of the objects they are looking at. Theoretical astronomers are more like physicists or mathematicians.  They think of a situation that might occur in outer space, write down all of the physics equations  that govern the system, and write computer programs to simulate what's going on. Then they can compare their results with real observations to see if they are correct! The last group of astronomers spend most of their time in labs, building and testing devices for other astronomers to use. This is a more hands on job, and takes just as much engineering skill as it does astronomy knowledge. If it weren't for these people building nice cameras and telescopes, astronomers would be out of a job!

Aside from doing actual science, astronomers spend a good amount of time writing papers about their findings, doing community outreach, and presenting their work at conferences and colleges around the world. Being an astronomer is a lot of work, but also a lot of fun. It's a fast paced and never ending job, and there is always more to learn about outer space!

How Old is that Star?

Determining the age of a star is not as easy as you might think. Since we can't ask a star how old it is, we have to guess the stars age by its appearance. And just like with humans sometimes looks can be deceiving! 

 

There are many ways to determine the age of a star, and today we will discuss stellar models. Like we've discussed before, stars can be placed on an Hertzsprung-Russel (HR) diagram. To do this, you need to measure the stars brightness, or luminosity, and you also need to know what type of star it is. Is it a big, hot blue star,? A cool, small, red? Somewhere in between? Astronomers can determine this by looking at a star's spectrum, or distribution of light, with a telescope. Once we know these two things, we can place the star at the proper position on the HR diagram. Astronomers have been hard at work modeling how stars form, and how their size, temperature, and brightness changes as they age. They have developed paths or lines that are placed on the HR diagram which show a stars path on the graph as it ages. There are models for before the star has reached the main sequence, and after. Basically what you do, is place the star on the HR diagram, see which line it is closest too, and that tells you the stars size and age. Here is an example of how this works. The graph above shows brightness vs. temperature, and models (solid lines) for stars of different masses. Stars, in theory, follow one solid line path going right to  left as it ages. The star represents the spot on the diagram where some arbitrary star's properties are. Based on its position, the star is probably about 4 times the mass of the sun, and about 200,000 years old! This is before it has started hydrogen burning, and is still a "baby" star. You can follow the same method with different models and estimate the age of a star that is burning hydrogen, or on its way towards death.

Discovery of Uranus' Rings

We have discussed before that all the gas planets in the solar system have rings.  Even through a small telescope Saturn has visible rings, but Jupiter, Uranus and Neptune do not. So how did astronomers discover their rings in the first place?

Hubble image of Uranus and its rings

The rings around Uranus were discovered in 1977. Astronomers knew that Uranus was going pass in front of a distant star in the night sky, from Earth's perspective. They pointed their telescopes to towards the planet each night, and expected to see the planet block the light from the star only when the star was directly behind the planet. What they actually observed was the star flickering right before and right after is passed behind the planet. This meant that there must be some unseen object near the planet blocking the starlight! The only plausible explanation was that Uranus has very thin, dim rings that are not visible from telescopes here on Earth. In 1986, Voyager flew by Uranus and imaged the rings for the first time, proving  their existence. Since then, we have discovered rings around Jupiter and Neptune in similar ways.

NASA Missions Extended

 

Artists conception of Spitzer, Planck and Kepler (left to right)

Astronomers received some great news a few days ago. Three major space telescopes, Kepler, Spitzer and Planck, have had their missions extended! This is great news, as astronomers will obtain more data and hopefully make some big discoveries! But what can we do with these telescopes?

The Kepler Space Telescope is an optical telescope has been actively searching for exoplanets. It looks at the same region of the sky 24/7, and measures the brightness of 150,000+ stars. If one of them dims for a short period of time, it might be due to a planet crossing in front of the star and blocking the light. Kepler has already found over 2000 potential exoplanets in the last 2.5 years of operation, and it's funding has been extended until 2016

The Spitzer Space Telescope is an infrared telescope that has been operating since 2004. For the telescope's detector to work properly, it needs to be kept extremely cold. Unfortunately, the cryogenics which keep it cool have run out, but the detector still functions, and some science can be done with the telescope. Astronomers have used Spitzer to look at young stars, distant galaxies, and many other objects that are "hidden" behind giant clouds of gas.  It will continue to operate for another two years.

Planck is a jointly funded NASA and ESA telescope which has been operational for about three years. It's a space based microwave/radio telescope whose main purpose is the study the cosmic microwave background. This is the first light emitted by the universe after the Big Bang. It will help us understand how the universe began by observing it right after it was born. Astronomers also use Planck to study distant galaxies, and objects in our solar system.

Image Credit:  NASA/JPL-Caltech

Finding the Planets

Today, we take a more observational approach to our astronomy lesson which will require you to go outside tonight and look at the stars. If  you've taken a look at the sky lately, you might have noticed a few extra bright objects up there. These bright objects are not really large stars, they are actually the planets in our solar system! Three of the planets (Venus, Mars and Jupiter) are visible just after sunset right now (assuming skies are clear where you are!) To find the planets, start by looking west. You should see two very bright objects in a straight line fairly low in the sky, brighter than any other stars around them. These are Venus (brightest one) and Jupiter! Once you've found them, turn around and look east. There should be another bright object in the sky that has a distinct red hue to it. That is Mars! Below are some images from Sky & Telescope magazine showing you where the planets are in relation to other stars and the moon. (Even though they say April 2nd they are about right for any day this week)

If you happen to have a telescope, or an observatory near by, take a look at these planets. If the night is very clear, you might be able to see the four Galilean moons of Jupiter or even the polar ice caps on Mars! It's really a spectacular sight!

Kepler's Third Law

The final law, Kepler's third law, is one of the most useful relations in astronomy. It states that the period of time it takes a planet to orbit the sun, squared (that's period*period), is proportional to its distance from the sun, cubed (distance*distance*distance). Or, as astronomers would say: P^2=a^3, where P is period and a is semi-major axis (i.e. distance).  The graph above shows the period and orbital distance of some planets in our solar system. The line going through all the points corresponds to the spot where P^2=a^3. The fact that all the planets fall on this line means that Kepler's third law is correct, and that we can predict the orbital time if we know the orbital distance, or vice versa. This relationship can be applied to most objects orbiting a larger object in space. Astronomers use it to estimate the period of exoplanets orbiting stars, and stars orbiting galaxy centers. 

And there you have it! Kepler's three laws of planetary motion!

Image Credit: Kevin Brown, Reflections on Relativity

Kepler's Second Law

Kepler's second law states:  The line joining the planet to the Sun sweeps out equal areas in equal intervals of time. 

 

This law is often referenced as the "law of equal areas" . So what does it mean? In the diagram above we have a planet going around the sun (or any star) following an elliptical path (as the 1st law states). When the planet is at point A, we draw an imaginary line towards the star. The planet continues to orbit the star, and lets assume one month passes. The planet is now at point B, and we draw another imaginary line towards the star. The area shaded in blue is the imaginary triangle in space that is created by the two lines we drew. We can calculate the area of this triangle because we know the length of the two lines we just drew. Now we repeat this scenario for when the planet is at points X and Y, and again it took the planet one month to go from point X to point Y. Notice that it traveled a much shorter distance on its orbit, and that the imaginary triangle we made is a lot thinner. But, again we know the length of the lines we drew, and if you calculate the area of this green triangle, you should get exactly the same amount as for the blue triangle! So in one month, the planet sweeps out a path of equal area!

Why is this the case? When the planet is closer to the star, it feels a stronger gravitational force from the star. The star sort of whips the planet around the corner closest to it, and has a weaker effect when the planet is farther away. All planets that orbit their host star in an ellipse will follow this rule.