We Come From the Stars

This is Our Home Galaxy, and a Couple of Neighbors

As the Milky Way rises over the horizon at the European Southern Observatory, its companion galaxies also come into view. The Large Magellanic Cloud (LMC) at far left lies about 160,000 light-years away, while the Small Magellanic Cloud (SMC, above and to the right of the LMC) lies about 200,000 light-years away. New simulations show that the LMC stole stars from the SMC when the two galaxies collided 300 million years ago. Microlensing events that have been observed are due to LMC stars passing in front of a stream of stars pulled from the SMC.
Credit: ESO/Y. Beletsky

When you look out at the night sky, you’re looking at our ancient home. Yes, Earth is our current home. But, in the grand scheme of things, the galaxy — and all the elements that make it are also our home.  The elements that make up our bodies, our planets, and our star all were either created in the Big Bang (hydrogen, for example), or inside other stars (carbon, oxygen, nitrogen, etc.).  Multiple generations of stars have lived and died in the galaxy, and we are the resulting “star stuff”.

But, there’s more than star stuff out there.  There are mysterious things that may tell astronomers more about types of matter in the cosmos and distribution of that matter throughout the universe.

Astronomers have been studying one of those two irregular-looking clouds of stars that appear just below our galaxy in this image to understand a category of objects called MACHO (Massive Compact Halo Objects). These were thought to be things about the mass of a star that were so faint they couldn’t be easily detected. Surveys of this region of our galactic neighborhood have been underway to see if MACHOs could be part of that mysterious collection of “stuff” called “dark matter” that seems to be an incredibly important part of the universe.

In order for MACHOs to make up dark matter, they must be very faint. To even decide if they’re “there”, astronomers looked for a phenomenon known as microlensing. During a microlensing event, a nearby object passes in front of a more distant star. The gravity of the closer object bends light from the star like a lens, magnifying it and causing it to brighten. If a MACHO does this, then they’d know a little bit more about the object.

By studying the LMC, astronomers hoped to see MACHOs within the Milky Way lensing distant LMC stars. The number of microlensing events observed by various teams was smaller than needed to account for dark matter, but much higher than expected from the known population of stars in the Milky Way. This left the origin of the observed events a puzzle and the existence of MACHOs as exotic objects a possibility.

Instead of MACHOs, a trail of stars removed from the SMC could well be responsible for the microlensing events. How do astronomers know this? They’ve done computer simulations showing that the most likely explanation for the observed microlensing events was an unseen population of stars removed by the LMC from its companion, the SMC. Foreground stars in the LMC are gravitationally lensing the trail of removed stars located behind the LMC from our point of view.

Although the evidence for the trail of lensed stars is persuasive, they haven’t been directly observed yet. That will take time, since these could be faint. A number of teams are searching for the signatures of these stars within a bridge of gas that connects the Magellanic Clouds. The computer models used to simulate the trail will point the way for astronomers to find the other “stuff” that makes up the galaxies… and intergalactic space.


The Hungry Black Hole

G2 Is Entering the Galactic House

Simulations of the dust and gas cloud G2 on its orbit around the Milky Way central black hole SgrA*.
Photo courtesy of M. Schartmann and L. Calcada/ European Southern Observatory and Max-Planck-Institut fur Extraterrestrische Physik.

The heart of our Milky Way Galaxy is home to a black hole named Sagittarius A* (known as Sag A* for short).  It contains the equivalent of three to four million times the mass of the Sun, and as black holes, it’s kind of a quiet, friendly one. By that, I mean that it’s not constantly gulping down massive amounts of material as some other galaxies’ black holes do.  Sure, it does eat, but it seems to do so only sporadically.

Astronomers and lab physicists at the Lawrence Livermore Labs have created a supercomputer simulation showing a gas cloud called G2 which is on its way to being Sag A*’s latest snack.

Three-dimensional, volume visualization spanning the period 2010 to 2020, of the gas and dust cloud as it approaches the Sgr A* black hole near the center of the Milky Way galaxy. (Courtesy Dr. P. Chris Fragile)

They came up with six simulations showing how the event might unfold as the cloud approaches the black hole. The cloud’s makeup is still not well-understood and astronomers are still trying to figure out where it came from in the first place. It first showed up in 2002, and observers rushed to chart its size and other characteristics. The cloud appears to have a fair amount of dust that is quite warm — 500 degrees Kelvin, which is about twice as hot as Earth’s surface. The gas entrained in the cloud is superheated hydrogen and is about twice as hot as the surface of the Sun.

This cloud could be a burp of gas from an old star that was starting to lose some of its mass. Or, as some astronomers have suggested, it might have been material that didn’t quite form a planet. And, now it’s heading toward the region of the black hole, poised for a close brush with the gravity well of the black hole sometime in September 2013. As it gets nearer, the cloud will get heated to incredibly high temperatures which will make it easy to spot using radio and x-ray telescopes on Earth.

It’s likely that not all of the cloud will get sucked into the realm of the black hole. A great deal of it may pass well outside of the “point of no return” near the black hole. But, it will get shredded apart by the close encounter with the black hole’s incredibly strong gravitational pull.

Astronomers will be able to watch the black hole dally with the cloud (like a cat dallies with a mouse) for a few years. The closest approach of the cloud will take a few months, and they should be able to track as the cloud is affected by its encounter and breaks apart.