The weight of the Milky Way

It’s heavy:

Measuring the total mass of our home galaxy is a tough puzzle. It’s difficult to see it all at once, buried as we are within one of its spiral arms. And there’s a huge portion of the Milky Way we can never see, since it’s made up of dark matter, which doesn’t emit light at all. So to get an accurate number, researchers need to weigh both the visible and invisible material that makes up the galaxy.

Now scientists have done just that, using new data from the Hubble Space Telescope combined with the Gaia spacecraft. This latest mass measurement of the Milky Way weighs in at 1.5 trillion times heavier than our sun.

Now, how heavy is the universe? Assuming the Milky Way is an average galaxy, and there are 200 billion galaxies in the universe, then all galaxies combined would weigh 300 sextillion (300,000,000,000,000,000,000,000) times the mass of the sun, and that’s not counting all the stuff drifting around between galaxies.

Confirmed! Monster black hole at center of Milky Way galaxy

The enormous object with a mass of millions of suns at the center of our galaxy, 26,000 light-years away from the Earth, has long been believed to be a supermassive black hole. Now we have proof:

Researchers used the European Southern Observatory’s sensitive GRAVITY instrument on the Very Large Telescope (VLT) to observe infrared radiation flares coming from the accretion disc around Sagittarius A* — the massive object at the center of our galaxy. Scientists think that most galaxies have a supermassive black hole at their center, but they’d never before had the data and observations to prove it.

To measure the effects of gravity near to a black hole, scientists needed to observe an object actually traveling close to it. They found their mark in a small star called S2 whose orbit takes it deep within Sagittarius A*’s gravity well every 16 years. As they watched, they saw three bright flares traveling around the black hole’s event horizon at about 30 percent of the speed of light — around 216 million miles per hour.

It’s exactly what Einstein’s theory of general relativity predicts would occur when a hot spot (like S2) passes close to a black hole weighing as much as 4 million suns, and the observation helps to confirm that it’s really there.

Never before have scientists observed material orbiting so close to a black hole’s event horizon.

Oh, and in case you were worried:

If the Milky was does in fact have a black hole in the center, will the entire Milky Way eventually be drawn in, (like vacuuming a sheet off of a bed), or are certain parts too far away? Is there a big sphere of empty space around a black hole?

No, the popular picture of a black hole as a huge vacuum cleaner sucking in everything around it is inaccurate. Black holes, even the one at the center of our galaxy, are very small. Only if you get very close to a black hole’s event horizon does it start pulling everything in. So no, most of the galaxy will not eventually fall into the hole. Whether black holes have empty space around them or not depends on their environment. There may be objects or gas close enough to fall in, or there may not be. Many black holes have disks of infalling material around their equators. –Dave Kornreich

Sagittarius A NASA

Supermassive black hole Sagittarius A* (Source: NASA)

A near miss

Sagittarius dwarf galaxy

Sagittarius dwarf galaxy (Hubble)

Scientists find that the Milky Way may have had a close brush with another galaxy in the relatively recent past (cosmically speaking):

A new study published Wednesday in Nature has tracked the motion of more than six million stars in our galaxy using the European Space Agency’s Gaia spacecraft. The research reveals that the Milky Way nearly collided with another nearby galaxy — called the Sagittarius dwarf galaxy — sometime in the past 300 to 900 million years. This cosmic “fender bender” set millions of stars moving like ripples on a pond, the authors say. […]

The shape wasn’t the only thing that surprised them. Scientists knew that the Milky Way has seen many collisions over its 13.5 billion year history. But they thought that was ancient history.