Starship Asterisk*

Centaurus A

Explanation: What's the closest active galaxy to planet Earth? That would be Centaurus A, only 11 million light-years distant. Spanning over 60,000 light-years, the peculiar elliptical galaxy is also known as NGC 5128. Forged in a collision of two otherwise normal galaxies, Centaurus A's fantastic jumble of young blue star clusters, pinkish star forming regions, and imposing dark dust lanes are seen here in remarkable detail. The colorful galaxy portrait is a composite of image data from space- and ground-based telescopes large and small. Near the galaxy's center, left over cosmic debris is steadily being consumed by a central black hole with a billion times the mass of the Sun. As in other active galaxies, that process generates the radio, X-ray, and gamma-ray energy radiated by Centaurus A.

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I was just thinking it's been a long time since we last had an APOD galaxy picture - and then we get this stupenduously gorgeous Robert Gendler, Roberto Colombari, Hubble and ESO portrait of the nearest galactic trainwreck, NGC 5128!

Ann

unknown unknowns wrote:These 'black holes' are amazing. How is it possible that no light signal goes out from behind the 'event horizon', but gravitation signals are emitted without problems and keep pulling junk in? Or is gravity superluminal? :roll:

Gravity is not superluminal. But there are no gravitational "signals" coming from inside the event horizon.

unknown unknowns wrote:These 'black holes' are amazing. How is it possible that no light signal goes out from behind the 'event horizon', but gravitation signals are emitted without problems and keep pulling junk in? Or is gravity superluminal?

It's not the black hole itself that shines, but the material swirling around it at very close to the speed of light in its spiral of death into the black hole.

Why doesn't the material trapped by the black hole "fall straight in" instead of circling around it faster and faster? First, some material may fall straight in. But in space, things normally move in circles (or rather in ellipses). The Earth isn't falling into the Sun, is it? It's circling around the Sun like the other planets. Even comets, in their extremley elongated orbits, are not typically falling straight into the Sun, even though some do.

So the objects that fall into black holes typically begin their journey of death by circling around the black hole, in tighter and tighter orbits. As the orbits get smaller, the objects move faster. Before they get so close that they actually fall in, they typically swirl around close to the speed of light.

But active black holes typically have accretion disks, made up of lots and lots of objects, all moving very fast, but moving at different speeds. The objects "chafe" against each other at terrific speeds, causing continuous little "explosions" and even releasing X-rays as they do so. Because of this, the accretion disk shines brilliantly, while the black hole itself is dark.

Ann

unknown unknowns wrote:

But there are no gravitational "signals" coming from inside the event horizon.

So how come it is a 'billion solar masses'? If no information can come from behind the 'event horizon', how come the space around it 'knows' how much mass it 'swallowed'? God keeping track? :lol2:

The deformation of space is neither information nor a signal.

I too wish to ask how come the black hole has a mass of a billion suns. When I clicked on the "active galaxy" link I found that, on Jan. 10th 2008, the same black hole had a mass of 10 million suns. This means its mass is now 100 times what it was less than eight years ago!

I have no patience for such rude guests.

ianboswell65 wrote:I too wish to ask how come the black hole has a mass of a billion suns. When I clicked on the "active galaxy" link I found that, on Jan. 10th 2008, the same black hole had a mass of 10 million suns. This means its mass is now 100 times what it was less than eight years ago!

My guess is that it's a typo in today's caption. The wikipedia page on Centaurus A says the mass of the central black hole is about 55 million solar masses. For comparison, the mass of the central black hole in the Milky Way is measured to be about 4 million solar masses. But some galaxies really do have central black holes with masses of several billion solar masses, see here.

Morning all! My question is regarding the phrase 'active galaxy'. I know that M31 (Andromeda) is the closest galaxy to us, but why is this galaxy considered the closest 'active galaxy'? What makes a galaxy 'active'? (And why is M31 not - or no longer - classified as active?)

And this is a beautiful image.

Indigo_Sunrise wrote:Morning all! My question is regarding the phrase 'active galaxy'. I know that M31 (Andromeda) is the closest galaxy to us, but why is this galaxy considered the closest 'active galaxy'? What makes a galaxy 'active'? (And why is M31 not - or no longer - classified as active?)

And this is a beautiful image.

An active galaxy is one that produces huge amounts of energy; they produce more energy than the total energy emitted from each of the stars in the nebula, this is extra energy come from mater being consumed by the black hole at its heart.

Chris Peterson wrote:

unknown unknowns wrote:
These 'black holes' are amazing. How is it possible that no light signal goes out from behind the 'event horizon', but gravitation signals are emitted without problems and keep pulling junk in? Or is gravity superluminal?

Gravity is not superluminal. But there are no gravitational "signals" coming from inside the event horizon.
The deformation of space is neither information nor a signal.

• This is certainly quite true for classical General Relativity.
  
  But it does make it all the harder to get one's head around the concept of gravitons.

https://en.wikipedia.org/wiki/Graviton wrote:
<<In physics, the graviton is a hypothetical elementary particle that mediates the force of gravitation in the framework of quantum field theory. If it exists, the graviton is expected to be massless (because the gravitational force appears to have unlimited range) and must be a spin-2 boson. It can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation. Seeing as the graviton is hypothetical, its discovery would unite quantum theory with gravity. This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton, so that the only experimental verification needed for the graviton may simply be the discovery of a massless spin-2 particle.

String theory predicts the existence of gravitons and their well-defined interactions. A graviton in perturbative string theory is a closed string in a very particular low-energy vibrational state.>>

neufer wrote:

Chris Peterson wrote:But there are no gravitational "signals" coming from inside the event horizon.
The deformation of space is neither information nor a signal.

• This is certainly quite true for classical General Relativity.
  
  But it does make it all the harder to get one's head around the concept of gravitons.

If I recall correctly, the interactions with gravitons are treated like interactions with photons. Just as the interaction between to electrons is described by an exchange of a photon, the gravitational interaction of a black hole with a test particle (e.g. a star) is described by the exchange of a graviton. The black hole is also treated as a point particle, and the graviton does not originate from inside the event horizon.

Indigo_Sunrise wrote:Morning all! My question is regarding the phrase 'active galaxy'. I know that M31 (Andromeda) is the closest galaxy to us, but why is this galaxy considered the closest 'active galaxy'? What makes a galaxy 'active'? (And why is M31 not - or no longer - classified as active?)

And this is a beautiful image.

An active galaxy has a supermassive central black hole that is actively feeding on material that is falling into it. Usually such a galaxy has some sort of accretion disk surrounding the black hole, and the inner part of the accretion disk is spiralling into the black hole.

Both the Milky Way and Andromeda have supermassive central black holes, but neither black hole is actively feeding on surrounding material.

Ann

Centaurus A is very low in the southern sky from my location, but it is large and bright and the central dust lane is easily visible. Under best conditions, some detail and structure is visible within the dust lane itself.

Thank you for your explanations, Paul and Ann. They definitely helped!

An Exquisite shot!

Are we viewing this Edge On?...or at a slight angle so we see the Top? The dust lanes make it look like Edge On...but if you look closely, there are star forming pink regions that look like they are..."on the other side of the galaxy"...
2 dimensional illusion???

Anybody know for sure???
Thanks.

:---[===] *

This is a fantastic image! What is immediately apparent to me is how clearly this very detailed image shows the various HII regions and emission nebulae!

Also it is likely that the Milky Way was an active galaxy in the past.

It was snack time at the center of the Milky Way some time back.

http://www.nytimes.com/2014/02/18/scien ... .html?_r=0

Then nothing seemed to become of it.

http://dujs.dartmouth.edu/physical_scie ... k4VyU-FOJB

I'm curious if the interaction will be more like this but I haven't seen any recent news.

http://www.nature.com/news/why-galactic ... op-1.15591

Time will tell.

This interesting thread includes suggestion that deformation of space does not send a signal .. yet space is said to host uncountable little quantum energy stuffs and Dark Matters and Dark Energies and quantum bits of stuff arising and being negated continuously .. so .. as all matter in space is in in motion through space thereby deforming space and everything IN space why would the deformation NOT send out a signal of some kind .. squeeze a wet sponge and out comes water. Squeeze space and out must come something. http://scienceblogs.com/startswithabang ... -combined/

In fact, squeeze space and perhaps space is heated by the squeeze. Perhaps the squeezing and deformation is the SOURCE of all those quantum thingies being created which then annihilate themselves. Here is a link which may or may not support space heat generated by matter squeezing space. https://books.google.com/books?id=V3rU4 ... ce&f=false I'm not a mathematician so don't know what the language means, outside of squeeze and space that is.

Very interesting image
Very interesting the explanations

Alohascope wrote:This interesting thread includes suggestion that deformation of space does not send a signal ...

That was not my suggestion. If you're reading what I said that way, you're misunderstanding me. What I said was that the static deformation of spacetime created by the mass of a black hole isn't a signal in the sense of conveying information. No information is escaping from inside the event horizon. That doesn't mean that a black hole doesn't dynamically distort spacetime resulting in the transmission of information via gravitational waves. But that can be understood without requiring any sort of "gravity" to cross the event horizon.

Cool name for a galaxy. Specially when spelled with a K. By the way, any animations of a galactic birth and aging ?Is it related to dark Matter becoming visible matter ?

Ron-Astro Pharmacist wrote:It was snack time at the center of the Milky Way some time back.
Then nothing seemed to become of it.
I'm curious if the interaction will be more like this but I haven't seen recent news.
Time will tell.

Latest news: http://chandra.harvard.edu/photo/2015/sgra2/

Same in video: http://chandra.harvard.edu/photo/2015/s ... lg_web.mov

ta152h0 wrote:Cool name for a galaxy. Specially when spelled with a K. By the way, any animations of a galactic birth and aging ?Is it related to dark Matter becoming visible matter ?

http://www.space.com/25878-spiral-galax ... video.html