Five Short Stories from Big Astronomy

A Peek at What Astronomers are Finding

 The Andromeda Galaxy is the closest spiral galaxy to the Milky Way. Adam Evans/Wikimedia Commons.

The science of astronomy concerns itself with objects and events in the universe. This ranges from stars and planets to galaxies, dark matter, and dark energy. The history of astronomy is filled with tales of discovery and exploration, beginning with the earliest humans who looked to the sky and continuing through the centuries to the present time. Today's astronomers use complex and sophisticated machines and software to learn about everything from the formation of planets and stars to the collisions of galaxies and the formation of the first stars and planets. Let's take a look at just a few of the many objects and events they are studying. 

Exoplanets!

 New research finds that exoplanets can be divided into three groups – terrestrials, gas giants, and mid-sized "gas dwarfs" – based on how their host stars tend to fall into three distinct groups defined by their compositions. All three are portrayed in this artist's conception. J. Jauch, Harvard-Smithsonian Center for Astrophysics.

 By far, some of the most exciting astronomy discoveries are planets around other stars. These are called exoplanets, and they appear to form in three "flavors": terrestrials (rocky), gas giants, and gas "dwarfs". How do astronomers know this?  The Kepler mission to find planets around other stars has uncovered thousands of planet candidates in just the nearby part of our galaxy. Once they're found, observers continue to study these candidates using other space-based or ground-based telescopes and specialized instruments called spectroscopes. 

Kepler finds exoplanets by looking for a star that dims as a planet passes in front of it from our point of view. That tells us the planet's size based on how much starlight it blocks. To determine the planet's composition we need to know its mass, so its density can be calculated. A rocky planet will be much denser than a gas giant. Unfortunately, the smaller a planet, the harder it is to measure its mass, especially for the dim and distant stars examined by Kepler.

Astronomers have measured the amount of elements heavier than hydrogen and helium, which astronomers collectively call metals, in stars with exoplanet candidates. Since a star and its planets form from the same disk of material, the metallicity of a star reflects the composition of the protoplanetary disk. Taking all these factors into account, astronomers have come up with the idea of three "basic types" of planets. 

Munching on Planets

 An artist's conception of what a bloated red giant star will look like as it gobbles up its closest planets. Harvard-Smithsonian Center for Astrophysics

Two worlds orbiting the star Kepler-56 are destined for stellar doom. Astronomers studying Kepler 56b and Kepler 56c discovered that in about 130 to 156 million years, these planets will get swallowed up by their star. Why is this going to happen?  Kepler-56 is becoming a red giant star. As it ages, it has bloated out to about four times the size of the Sun. This old-age expansion will continue, and eventually, the star will engulf the two planets. The third planet orbiting this star will survive.  The other two will get heated, stretched by the star's gravitational pull, and their atmospheres will boil away.  If you think this sounds alien, remember: the inner worlds of our own solar system will face this same fate in a few billion years. The Kepler-56 system is showing us the fate of our own planet in the distant future! 

Galaxy Clusters Colliding!

 Colliding galaxy clusters MACS J0717+3745, more than 5 billion light-years from Earth. Background is Hubble Space Telescope image; blue is X-ray image from Chandra, and red is VLA radio image. Van Weeren, et al.; Bill Saxton, NRAO/AUI/NSF; NASA

In the far distant universe, astronomers are watching as four clusters of galaxies collide with each other. In addition to mingling stars, the action is also releasing huge amounts of x-ray and radio emissions. The Earth-orbiting Hubble Space Telescope (HST) and Chandra Observatory, along with the Very Large Array (VLA) in New Mexico have studied this cosmic collision scene to help astronomers understand the mechanics of what happens when galaxy clusters crash into each other. 

The HST image forms the background of this composite image. The x-ray emission detected by Chandra is in blue and radio emission seen by the VLA is in red. The x-rays trace the existence of hot, tenuous gas that pervades the region containing the galaxy clusters. The large, oddly-shaped red feature at the center probably is a region where shocks caused by the collisions are accelerating particles that then interact with magnetic fields and emit the radio waves. The straight, elongated radio-emitting object is a foreground galaxy whose central black hole is accelerating jets of particles in two directions. The red object at bottom-left is a radio galaxy that probably is falling into the cluster.

These kinds of multi-wavelength views of objects and events in the cosmos contain many clues about how collisions have shaped the galaxies and larger structures in the universe. 

A Galaxy Glitters in X-ray Emissions!

 A new Chandra image of M51 contains nearly a million seconds of observing time. X-ray: NASA/CXC/Wesleyan Univ./R.Kilgard, et al; Optical: NASA/STScI

 There's a galaxy out there, not too far from the Milky Way (30 million light-years, just next door in cosmic distance) called M51. You might have heard it called the Whirlpool. It's a spiral, similar to our own galaxy. It differs from the Milky Way in that it is colliding with a smaller companion. The action of the merger is triggering waves of star formation. 

In an effort to understand more about its star-forming regions, its black holes, and other fascinating places, astronomers used the Chandra X-Ray Observatory to gather up x-ray emissions coming from M51. This image shows what they saw. It's a composite of a visible-light image overlaid with x-ray data (in purple). Most of the x-ray sources that Chandra saw are x-ray binaries (XRBs). These are pairs of objects where a compact star, such as a neutron star or, more rarely, a black hole, captures material from an orbiting companion star. The material is accelerated by the intense gravitational field of the compact star and heated to millions of degrees. That creates a bright x-ray source. The Chandra observations reveal that at least ten of the XRBs in M51 are bright enough to contain black holes. In eight of these systems the black holes are likely capturing material from companion stars that are much more massive than the Sun.

The most massive of the newly formed stars being created in response to the upcoming collisions will live fast (only a few million years), die young, and collapse to form neutron stars or black holes. Most of the XRBs containing black holes in M51 are located close to regions where stars are forming, showing their connection to the fateful galactic collision. 

Look Deep into the Universe!

 Hubble Space Telescope's deepest view of the cosmos, uncovering star formation in some of the earliest galaxies in existence. NASA/ESA/STScI

Everywhere astronomers look in the universe, they find galaxies as far as they can see. This is the latest and most colorful look at the distant universe, made by the Hubble Space Telescope.

The most important outcome of this gorgeous image, which is a composite of exposures taken in 2003 and 2012 with the Advanced Camera for Surveys and the Wide Field Camera 3, is that it provides the missing link in star formation. 

Astronomers previously studied the Hubble Ultra Deep Field (HUDF), which covers a small section of space visible form the southern hemisphere constellation Fornax, in visible and near-infrared light. The ultraviolet light study, combined with all the other wavelengths available, provides an image of that part of the sky that contains about 10,000 galaxies. The oldest galaxies in the image look as they would just a few hundred million years after the Big Bang (the event that began the expansion of space and time in our universe).

Ultraviolet light is important in looking back this far because it comes from the hottest, largest, and youngest stars. By observing at these wavelengths, researchers get a direct look at which galaxies are forming stars and where the stars are forming within those galaxies. It also lets them understand how galaxies grew over time, from small collections of hot young stars.