Starship Asterisk*

Today's APOD stated the Centaurus A galaxy was the closest active galaxy to Earth at 11 million lys. Yesterday they said the the IC 342 was 7 million lys away and active. Am I missing something here? Also, if IC 342 is the closest, we should name it. I suggest the Octopus Galaxy as it appears to have eight arms.

http://antwrp.gsfc.nasa.gov/apod/ap080110.html

The key word is "active". As in active galactic nucleus. IC 342 isn't considered one of those.

The IC 342 writeup describes "star formation activity." You contrast this to "active galactic nucleus." One is considered active, one not. Please elaborate.

Yes, we should definitely name it. Octopus works for me.

BTW, I discovered APOD only last June when I was trying to find out something about auroras. Have found it an exciting combination of stunningly beautiful and immensely educational. It's often the last place I look before leaving my computer and going to bed. Have had many questions, which mostly I research through the links (sometimes keeping me up for hours!) Just joined this discuss site...am finding it fascinating, and sometimes highly humorous...love the repartee. Am hoping no question is considered stupid. Hence the above. But it isn't obvious to me how to include a reference to what an earlier post has stated. Easy to explain?

Cherie

Cherie wrote:The IC 342 writeup describes "star formation activity." You contrast this to "active galactic nucleus." One is considered active, one not. Please elaborate.

A galaxy hosting an AGN is called an active galaxy. That refers to the activity of broadband radiation from the accretion disk of a supermassive black hole in the center (nucleus). Many (all?) such galaxies have jets streaming out perpendicular to the disk.
Star formation activity is hot bright young stars being born from dust and nebulae, often found in the arms of a galaxy, away from the nucleus.

Cherie wrote:Am hoping no question is considered stupid. Hence the above.

"The only stupid question is the one left unasked," a wiser man than me once said.

Cherie wrote:But it isn't obvious to me how to include a reference to what an earlier post has stated. Easy to explain?

Try the Quote button at the top right of each post.

Many (all?) such galaxies have jets streaming out perpendicular to the disk.

But IC 342 does not?

Still confused. But maybe it's just semantics combined with my lack of knowledge.

Oops, still don't have the hang of the quote thing.

A clarification on Case's reply.

Not all, in fact I think only a small percentage of galaxies, have large amounts of radiation being emitted from their nucleus, or center. A great number, the Milky Way included, emit only in amounts too small to be noticed, or perhaps not at all.

These active galactic nuclei (AGN) have super massive black holes at their center which are rapidly pulling in gas and dust that surrounds the core. As the matter falls into the black hole, it heats up, glows, and forms the x-ray and particle jets (black holes are messy eaters!) that astronomers have noticed and on account of which classify the galaxy as active. The black hole at the center of the Milky Way is though to have consumed all the nearby gas and dust billions of years ago, so it is now silent, although we know it's there because it's gravity affects stars near the center.

IC 342, like the Milky Way, does not have an active galactic nuclei. However, it does show a lot of star formation, which is an entirely separate form of activity.

Admittedly, the word choice is a little confusing. An active galaxy is fairly explicitly defined, but it's not the only use of the word active in astronomy.

Hope that clears it up.

Thanks, Lucky...and Case, too. With these explanations, I will review writeups of the last 25 or so galaxies featured on APOD and see if I can tell the difference between those with active nuclei (sp?) and those without.

Thank you all for clarifying this subject for me and to Cherie for following through the question. From this I would assume that the Andromeda galaxy has an inactive core also.

astrowino wrote:Thank you all for clarifying this subject for me and to Cherie for following through the question. From this I would assume that the Andromeda galaxy has an inactive core also.

Correct. Although in 2.5 billion years when the Milky Way and Andromeda run into each other, the gravitational interactions may fling enough matter into the center that they'll once again become active. I'm not sure I'd say that's a good thing, though.

Well, I for one am waiting with bated breath for the Big Collision.

Nah, I guess not. With my luck they'll discover how to make people live forever right after I'm gone. Or maybe when I'm old and senile.

I can't help but wonder what it's like inside a black hole. A "hole" 'nother universe, perhaps?

-Noel