Know Your Galactic Neighbors

[Brown]

A couple of pieces of news about our galactic neighbors.

First, according to Reuters and Yahoo, our own galaxy is in the process of "eating" another galaxy, which was something of a surprise to me, as I had no idea that our galaxy was hungry.

On its way to oblivion, the dwarf Sagittarius -- which is about 10,000 times the mass of the Milky Way -- is getting stretched, torn apart and ultimately eaten, scientists at the University of Virginia and the University of Massachusetts reported.

"It's clear who's the bully in the interaction," Steven Majewski of the University of Virginia, lead author of the report, said in a statement.

The bully is, well, us. The Milky Way.

Second, the sky was fairly clear and I had a pretty good view of the Andromeda Galaxy tonight. If you know where to look, you can see this galaxy with your naked eye, and you can see it even better with binoculars.

 

[uneasy]

Wasn't this the premise of EE "Doc" Smith's Lensman series?

I saw this news earlier today, and it confused me. What I don't understand is how can our galaxy be eating another one that is 10000 times more massive than ours? Is that what it says? If that is the case, wouldn't that galaxy be eating ours?

Or is the media trying to avoid a panic.

 

[Brown]

What I don't understand is how can our galaxy be eating another one that is 10000 times more massive than ours? Is that what it says? If that is the case, wouldn't that galaxy be eating ours?

Yeah, I wonder about this, too. I just assumed that the author had it backward, and that the dwarf galaxy has 1/10000 of our galaxy's mass. But this really isn't clear.

 

[uneasy]

Yeah, I wonder about this, too. I just assumed that the author had it backward, and that the dwarf galaxy has 1/10000 of our galaxy's mass. But this really isn't clear.

I chalked it up to bad journalism. I get so frustrated at news stories on science that I almost assume they are innacurate to start.

Galaxies are a fun thing to talk about with people though. Many people don't understand them (never saw Cosmos I guess), and if you can explain it to them, it's a joy to see them realize how big things are.

 

[Rayn]

I've read some on super-massive black holes that are at the core of many galaxies. I believe our galaxy is one of these galaxies. What I'm unsure of is how a star is more massive than a super-massive black hole? Is that possible? (going on the assumption that maybe these journalists were right)

 

[wollery]

Sadly this is just another case of bad reporting.
The Sagittarius Dwarf Galaxy is, as the name suggests, pretty small in the galaxy arena. Before the tidal interaction with the Milky Way which stretched it out it was probably what's called a Dwarf Spheroidal galaxy with a mass of about 10exp7 to 10exp8 solar masses. The Milky Way on, the other hand, is pretty large as galaxies go and has a mass of 3.4exp11 solar masses.

In answer to Rayns question - the Milky Way does indeed have a supermassine black hole at the centre called Sagittarius A* which is believed to have a mass of about 2.6exp6 solar masses, and no, no star could possibly have a mass anywhere near this. The theoretical limit on the mass of a star is about 100 solar masses and stars this size are extremely rare.

 

[wollery]

Okay, just read one of the papers, there are four (none of which have yet been accepted for publication in a peer reviewed journal, but having read one I'm sure that's just a formality).
They've used computer modelling of the galaxy interaction to find the best fit for the parameters of the Sagittarius Dwarf galaxy. They get a current mass of 3x10exp8 solar masses, which suggests that before the interaction started it had a mass of about 3.5x10exp8 solar masses - almost exactly 1/10,000th of the mass of the Milky Way.

 

[CurtC]

wollery, your notation is confusing me. I thought at first that when you said "10exp7 to 10exp8," which I first read as "ten ee times pee seven," then I thought that you mean 10^7 to 10^8. Then you said 3.4exp11, which can't be 3.4^11, because nobody raises 3.4 to the 11th power. Now you say 3x10exp8, which looks more like scientific notation. Now I think you're switching around, with three different notation schemes.

The "e" notation is pretty universally understood I think, so these figures would be 1e7 to 1e8, 3.4e11, and 3e8.

 

[uneasy]

So the reporter was off by a factor of 100000000. But who's counting?

 

[MRC_Hans]

Well, certainly not reporters. I doubt most of them even know how.

Hans