Author: Nicholos Wethington

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How many are in the Milky Way, though? There could be billions, many of them habitable and Earth-like, according to some astronomers. Our ability to detect planets orbiting other stars has been around for less than 20 years, and most of the planets discovered to date are much larger than Jupiter (in fact, extrasolar planets are commonly measured in “Jupiter masses”.)

There are a few different methods for detecting exoplanets. The primary techniques are astrometry and radial velocity measurements. Astrometry is basically measuring the gravitational influence of a planet as it orbits its star. How much it pulls the star side to side can give a lot of information as to the amount of mass the planet has. Measuring radial velocity is much like astrometry, only with this method the amount the star moves toward and away from the Earth is measured by observing the Doppler shift of the light coming from the star.

Another technique is called the transit method. As a planet orbits in front of its star, the light coming from the star is dimmed, and by observing the star for long periods of time, and taking the spectrum of the light both when the planet is in front of the star and behind, much can be known about the makeup of the planet’s atmosphere (if there is any). The transit method is often used in combination with astrometry and radial velocity measurements to estimate the mass of the planet.

Other methods for detecting planets are explained on the European Space Agency’s website and Curious About Astronomy. If you want a complete list of all planets detected so far, NASA’s PlanetQuest site is a great place to start, as well as The Extrasolar Planets Encyclopaedia.

Direct imaging of extrasolar planets is very difficult, as the overwhelming amount of light coming from the star a planet is orbiting completely washes it out. However, Hubble has imaged the planet Fomalhault b, and the system HR8799, which consists of three planets, was imaged using the Keck and Gemini telescopes.

There are currently a number of NASA missions working on the discovery of extrasolar planets, including Hubble, and the Spitzer Space Telescope. The Kepler mission, launched on March 6th of 2009, will monitor a section of the sky containing over 100,000 stars and use the methods described above in an effort to detect an exoplanets in that region. The Terrestrial Planet Finder mission is another mission to study all aspects of extrasolar planets in rather great detail, though it is still in the concept phase as of this writing.

Exoplanets were discussed on Astronomy Cast in  Episode 34: Discovering Another Earth and Episode 125: A Zoo of Extrasolar Planets.

Source: NASA

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If you’ve seen an image of the Milky Way from above or below, you will certainly notice that it has a spiral structure. Not all galaxies are created equal, though, as there are many, known as elliptical galaxies, that are blob-like, while others have irregular shapes. Ours is of a class of galaxies called barred spirals, because it has a rectangular bar in the middle of the galactic disk.

The Milky Way has four main spiral arms: the Norma and Cygnus arm, Sagittarius, Scutum-Crux, and Perseus. The Sun is located in a minor arm, or spur, named the Orion Spur. The galactic disk itself is about 100,000 light years across, and the bar at the center is estimated to be about 27,000 light years long.

Why is the Milky Way a spiral? This is due to its rotation, or rather, the rotation of matter inside the galactic disk around the center. It’s not as if the stars themselves stay in the spiral arms, and rotate around the center of the galaxy, though: if they did this, the arms would wind in tighter and tighter over time (2 billion years or so), since the stars in the center revolve faster than those further out.

The spirals are actually what is called a density wave or standing wave. The best way to describe this is the analogy of a traffic jam: cars travel on a busy road in a city, bunching up in jams over the course of a day at certain sections. But the cars move through the jam eventually, and other cars pile up behind them in the jam. The wave is at a certain location, with bunches of matter piling up there for a while, then moving on to be replaced by other matter. As dust and gas is compressed in the spirals, it is heated up and results in the formation of new stars. This star formation makes the trailing edge of the spiral brighter, and places the density wave “ahead”, where dimmer, redder stars are starting to be compressed.

When you see an image of the Milky Way like the one above, it’s not actually a photo of our galaxy. Since we inhabit the disk and have no way (currently) of going above or below, images of the Milky Way are generated by computers or artists. Astronomers have determined that the Milky Way is a spiral galaxy by mapping the movements of stars and hydrogen clouds in the disk.

The Milky Way is far from being the only spiral galaxy in the Universe. To view images of other spiral galaxies, go to the aptly-named Spiral Galaxies website, or NASA’s Astronomy Picture of the Day Spiral Galaxy Index.

To learn more about the Milky Way, check out Episode 99 of Astronomy Cast, or visit the rest of the Milky Way section in the Guide to Space.

Source: University of Wisconsin-Madison News

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If you look up into the night sky on a very clear night, in an area with very little light pollution, you will see a band of stars splashed across the sky. That band is the Milky Way, the spiral galaxy in which our Solar System lies, and where almost every object you can see with your naked eye calls home.

The Solar System is inside the disk of the Milky Way, and orbits in one of the spiral arms at 26,000 light years from the center of the galaxy. We can’t see the spiral structure of the galaxy from our planet because we are inside the disk and have no means of taking images from above or below the galaxy. Images of the Milky Way’s spiral structure are created from computer modeling based on information from stars as they orbit the galaxy.

Much of the Milky Way is invisible to us because we have to look through the plane of its disk – a lot of the Milky Way is on the other side of the galaxy, and there is so much dust and so many bright stars closer to us that we can’t see the stars behind all of this matter. Of the 5,000 to 8,000 stars in the Milky Way visible to the human eye from Earth, one can usually only see about 2,500 at a time. In fact, the few thousand stars we can see of the Milky Way with our naked eye are only about 0.000003% of the 200-400 billion stars that inhabit the spiral!

To see a picture of the entire Milky Way from the surface of the Earth at once, you have to create a mosaic of photographs taken at different times. This is because the Milky Way moves overhead at night with the rotation of the Earth, so can’t be viewed all at once from one spot. Many panoramas of our galaxy can be found on the web, but here’s a few to get you started:  NASA’s Astronomy Picture of the Day, the Spitzer Space Telescope’s very detailed, very large (55-meters long when printed) mosaic available for your perusal here – it’s a large image, so give it a little time to load – and a drawing by Knut Lundmark of over 7,000 stars in the Milky Way made in the 1950s.

To learn more about the Milky Way, visit the rest of the section here at the Guide to Space, listen to Episode 99 of Astronomy Cast, or visit the Students for the Exploration and Development of Space.

Source: NASA

Billions of years from now, the Milky Way will look totally different, as pictured to the left. When you look up at the sky, you may see another entire galaxy passing through the plane of our own, creating stars and supernovae and changing the entire sky.

Though most galaxies are rushing away from us as the Universe expands, Andromeda and the Milky Way are orbiting each other and closing in fast. Collisions between galaxies aren’t always catastrophic (the Milky Way is colliding with the Canis Major Dwarf galaxy right now, swallowing its stars up into the galactic disk), but they can trigger star formation on large scales and increase the number of supernovae.

When the galaxies collide, there is little chance that many stars will slam into each other directly because they are so spread out; however, the gas that lies between the stars can collide, heat up and trigger the formation of new stars. This interstellar gas and dust could also get sucked up by existing stars, increasing their mass to the point where they go supernova.

By the time the Milky Way and Andromeda collide, though, much of the gas in both galaxies will have been used up to create stars, so a “starburst” won’t happen. It won’t be a quick merger, though, and the spiral structure of each will be seriously changed. As you can see in this animated simulation from University of Toronto astronomer John Dubinski, the galaxies will pass through each other a few times, and the gravitational disturbance of this passage will throw stars willy-nilly into empty space.

Andromeda and the Milky Way will pass through each other once, then fall apart for about a billion years, then pass again, and again until finally settling down to merge completely about 5 billion years from now. The resulting galaxy won’t look anything like either of the merged galaxies – it will be a fuzzy blob called an elliptical galaxy. “Milkomeda” has a nice ring to it, and is one proposed name for the new merged galaxy.

When the galaxies do finally merge, there is a small chance that the Solar System will either join the Andromeda galaxy for a short while during one of the passes, or that it will be flung out of our galactic disk into interstellar space. For an in-depth analysis of this collision and statistics on the chances of the Sun and planets being ejected, check out “The Collision Between The Milky Way and Andromeda”  by Harvard-Smithsonian astrophysicists T.J. Cox and Abraham Loeb.

Of course, Andromeda is not the only thing that could collide with the Milky Way. There is currently a large cloud of hydrogen gas on a collision course with the Milky Way and though the edge of the cloud is already interacting with our galaxy, it won’t set off star-forming fireworks until at least 40 million years from now. Named Smith’s Cloud after the astronomer who discovered it in 1963, it is 11,000 light-years long and 2,500 light-years wide, and has enough hydrogen to form a million stars the mass of the Sun. More information about this collision can be found right here on Universe Today, and from the National Radio Astronomy Observatory.

Pamela and Fraser talk about what the Milky Way and Andromeda collision will look like in the September 28th, 2008 episode of Astronomy Cast, and the Milky Way in Episode 99.

Source: NASA

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The Andromeda galaxy is the closest spiral galaxy to the Milky Way (though it’s not the closest galaxy). It’s the most distant object you can easily see with the naked eye (under good observing conditions). Andromeda is 220,000 light years in diameter, and is one of the 35 objects that make up what is called the Local Group. Andromeda lies, of course, in Andromeda constellation.

The Andromeda galaxy (also known as  Messier 31, M31, or NGC 224) could be considered the big brother of the Milky Way, as it contains over a trillion stars (compared to our 200-400 billion), and is approximately 220,000 light years across to our 100,000. Andromeda and the Milky Way formed at roughly the same time – 13.5 billion years ago – near the beginning of the Universe. Our galaxy is thought to look much like Andromeda. Both Andromeda and the Milky Way got to their current size by eating up other galaxies they collide with. The expansion of the Universe causes most galaxies to move away from us, but Andromeda and the Milky Way are actually headed towards each other.

Andromeda and the Milky Way are good neighbors, but eventually our neighbor is going to move in with us – the Milky Way and Andromeda are approaching each other at 200 kilometers per second, and will eventually collide. There’s no need to panic, though, as Andromeda is over 2 million light years away, and the collision won’t happen for another 2 or 3 billion years. Astronomer John Dubinski of the University of Toronto has an excellent animated simulation from multiple perspectives of what this galactic dance could look like.

The collision between Andromeda and the Milky Way won’t be catastrophic, and after about 5 billion years from now the resulting galaxy will have settled down into an elliptical galaxy. There is a small chance, though, that the Sun won’t be part of this new “Milkomeda” galaxy.

Fraser and Pamela discuss how the collision between the Milky Way and Andromeda will look from Earth in the September 28th, 2008 episode of Astronomy Cast, and the Milky Way in Episode 99.

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The mass of the Milky Way depends on what you consider this question to mean. If you are only talking about the visible part of the Milky Way – all the stars, gas and dust that make up the disk – then the Milky Way’s mass is between 200-600 billion times that of the Sun. We can’t just put the Milky way on a bathroom scale to get this number, however. This number is reached by counting the number of stars in the galaxy and assuming their mass is roughly that of the Sun. The mass varies depending on where one defines the edge of the Milky Way to be.

But there is another way to check the heft of the Milky Way – by measuring how fast stars are rotating around the disk, the mass of the disk itself can be determined. In other words, the heavier the Milky Way is, the more of an effect gravity will have on the rotation, and the faster the stars will move through the disk. This number comes up to be a whopping 1-2 trillion times the mass of the Sun!  The most recent estimate from a study using information from the Sloan Digital Sky Survey measuring the velocity of over 2,4oo stars put the mass of the Milky Way and its halo at 1 trillion solar masses. Though astronomers don’t use kilograms when measuring such large objects as the Sun or galaxies, the Milky Way and its halo would be about 6 x 10^42 kilograms.

Where is all of this matter, if not in the stars? As with many contemporary mysteries in astronomy, the answer is dark matter. The Milky Way is thought to be home to a halo of dark matter – matter that cannot be detected except through its gravitational influence – which makes up approximately 80-90% of its mass. That’s right, the mass of the Milky Way that can be seen (through visible, X-ray, infrared, etc.) makes up only about 10-20% of its mass. This halo may extend out to as far as 300,000 light years from the galactic center.

For more information about the Milky Way, you can refer to Episode 99 of Astronomy Cast, visit the rest of our section here in the Guide to Space, or Swinburne Astronomy Online.

Source:
Sloan Digital Sky Survey