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Recent Podcast
Tour: Astronomers Spy Quartet of Cavities From Giant Black Holes
Tour: Astronomers Spy Quartet of Cavities From Giant Black Holes
Scientists have found four enormous cavities, or bubbles, at the center of a galaxy cluster using NASA's Chandra X-ray Observatory. (2021-12-16)

In A Galaxy Far, Far Away and Also Those Nearby

NASA: We have booster ignition and liftoff of Columbia, reaching new heights for women and X-ray Astronomy.

Martin Elvis: The main thing Chandra does is take these superb, sharp images.

Narrator: "In a galaxy far, far away..." These are some of the most famous words in movie history. But what do we already know about galaxies, and what do astronomers, like those using the Chandra X- ray Observatory, still hope to learn about them?

Let's start with the basics. All galaxies have certain things in common. Every galaxy is made up of some configuration of millions or even billions of stars, dust, gas, and the mysterious stuff known as dark matter. And all of this is bound together by gravity.

Of course, nothing in the Universe is ever that simple. There are billions of galaxies strewn across the cosmos, just as there are grains of sand on a beach. But astronomers have learned many things about galaxies, including how they fall into different kinds of categories. Dr. Roy Kilgard studies galaxies, and explains the different forms they come in.

Scientist: Historically, galaxies have been classified by their apparent shape as seen in optical telescopes. The 3 main categories are spiral, elliptical and irregular galaxies. Spiral galaxies have majestic, sweeping arms of stars and dust. These galaxies have a core, or bulge, of older stars and are actively forming new stars. Our own Milky Way is a spiral galaxy. Elliptical galaxies appear as rounded balls of light. These galaxies tend to have little on-going star-formation, so are composed of mostly older stars. Irregular galaxies have peculiar shapes that may be caused by gravitational interaction with nearby galaxies. This interaction can sometimes stir up rapid star- formation in an event called a starburst.

Narrator: Like most astronomical objects, galaxies were first studied in optical light. When astronomer Edwin Hubble saw galaxies through his telescope in the early 20th century, he called them "island universes". But telescopes that look at different types of radiation, like Chandra and its study of X-ray light, can help astronomers learn more about the galaxies than just meets the eye.

Scientist:With X-ray telescopes, we don't see the normal stars and dust that make up most of a galaxy. Instead, we see the most energetic objects: X-ray binary stars and supernova remnants. These objects appear as points of light to Chandra, and are often embedded in diffuse X-ray emission from hot gas. X-ray binaries are star systems with a black hole or neutron star accreting matter from a more normal star. By combining X-ray observations with those from optical telescopes like the Hubble Space Telescope, we can tell whether each X-ray binary is associated with young or old stars. The properties of the X-ray source associated with young stars can tell us about the star-formation history of a galaxy, whereas those associated with old stars can tell us about the distribution of matter throughout that galaxy.

With Chandra, we can determine the temperature and chemical composition of the hot gas in galaxies. This can tell us about the properties of the parent stars that produced the gas in supernova explosions. Studying the gas provides us with an independent measure of the star-formation in galaxies.

Narrator:So scientists have used Chandra and many other telescopes since Edwin Hubble's time to discover much more about galaxies, but they are by no means at the end of their journey. Roy Kilgard talks about some of the most pressing questions still open in the quest to understand galaxies.

Scientist: When we study galaxies, ultimately what we are studying is star formation: how stars are born, how they live and evolve, how they interact with one another, and how they die. In the X-ray, we are mostly concerned with the end of this chain: with the supernova remnants produced by the death of stars and with compact objects produced at the end of a star's life--the black holes and neutron stars we see in X-ray binaries. By combining the X-ray observations with other wavelengths, we can attempt to see the complete picture, from the youngest stars in blue and ultraviolet light to older stars in red and infrared wavelengths. By trying to understand this process, we are ultimately attempting to understand our own place; in our own Milky Way galaxy and in the universe as a whole.

Narrator:So the next time you hear a science fiction movie mention the far reaches of the galaxy, take a second to think about what that means. A galaxy is certainly a large and complicated object, but remember that it is just one of billions we know about in our Universe. Astronomers are using all of the tools available to them, including the Chandra X-ray Observatory, to learn more about galaxies far, far away, and also those nearby.

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