Astronomy Picture of the Day |
APOD: 1999 October 17 - Black Holes in Galactic Centers
Explanation:
Do all galaxies have black holes at their centers?
Although not even a single
galaxy
has yet been proven to have a central black hole,
the list of candidates continues to increase.
Results by astronomers using instruments like the
Hubble Space Telescope
now indicate that most - and possibly even all - large galaxies
may harbor one of these dense beasts.
In all the galaxies studied, star speeds continue to increase
closer the very center. This in
itself indicates a center millions of times more massive than
our Sun is needed to contain the stars.
This mass when combined with the limiting size make the
case for the central black holes.
Will we ever know for sure?
APOD: 1997 January 15 - Black Hole Signature from Advective Disks
Explanation:
What does a black hole look like? If alone,
a black hole
would indeed appear
quite black, but many black hole candidates are part of binary star systems.
So how does a black hole binary system
look different from a neutron star binary system?
The above drawings indicate it
is difficult to tell! Recent theoretical work,
however, has provided a new way to tell them apart: advective accretion flows (ADAFs).
A black hole system so equipped
would appear much darker than a similar neutron star
system. The difference is caused by the hot gas from the ADAF disk
falling through the event horizon
of the black hole and disappearing - gas that would have emitted
much light were the central object only a neutron star. Recent observations
of the soft X-ray transient
V404 Cyg
has yielded a spectrum
much like an ADAF onto a black hole
- and perhaps brighter than allowable from an ADAF onto a neutron
star.
APOD: 2000 December 10 - Too Close to a Black Hole
Explanation:
What would you see if you went right up to a
black hole?
Above are two computer generated images
highlighting how strange things would look.
On the left is a normal star field containing the
constellation Orion.
Notice the three stars of nearly equal brightness that make up
Orion's Belt.
On the right is the same star field but this time with a
black hole superposed in the center of the frame.
The
black hole has such strong
gravity that light is noticeably bent towards it -
causing some very unusual
visual distortions.
In the distorted frame, every star in the normal frame
has at least two bright images - one on each side of the
black hole.
In fact, near the
black hole, you can see the whole sky -
light from every direction is
bent around and comes back to you.
Black holes are thought to be the densest state of matter,
and there is indirect evidence for their presence in
stellar binary systems
and the centers of
globular clusters,
galaxies, and
quasars.
Authors & editors:
Robert
Nemiroff
(MTU)
& Jerry Bonnell (USRA)
NASA Technical Rep.:
Jay Norris.
Specific rights apply.
A service of:
LHEA at
NASA/
GSFC
&
Michigan Tech. U.