Astronomy Picture of the Day |
APOD: 1999 December 19 - Accretion Disk Binary System
Explanation:
Our Sun
is unusual in that it is alone - most
stars occur in multiple or
binary systems.
In a binary system, the
higher mass star will evolve faster and will eventually
become a compact object - either a
white dwarf star, a
neutron star, or
black hole.
When the lower mass star later
evolves into an expansion phase,
it may be so close to the compact star that its
outer atmosphere actually falls
onto the compact star.
Such is the case
diagrammed above. Here
gas from a blue
giant star is
shown being stripped away into an accretion disk around its compact binary
companion.
Gas in the
accretion disk swirls around, heats up, and
eventually falls onto the compact star.
Extreme conditions frequently occur on the surface
of the compact star as gas falls in, many times
causing detectable
X-rays,
gamma-rays, or even
cataclysmic novae explosions.
Studying the extreme conditions in these
systems tells us about the inner properties
of ordinary matter around us.
APOD: 1997 February 19 - Mizar Binary Star
Explanation:
Mizar (sounds like "My Czar") is a binary star.
In fact, most stars are binary stars.
In a
binary star system,
each star of the pair follows an elliptical orbital path.
Mutual gravity causes the stellar companions to
glide around their orbits
as if tied to the ends of an elastic string
passing through a balance point between them.
The balance point is the system's "center of mass".
Also known as
Zeta Ursa Majoris,
Mizar is the middle star in the
handle of the Big Dipper and at a distance of 88 light years, was
the first binary star system
to be imaged telescopically.
Spectroscopic observations of the Mizar system show periodic
doppler shifts, revealing that
both stars, Mizar A and Mizar B, are themselves binary stars!
But, the companions are too close to be directly observed as separate stars,
even by the largest telescopes.
In developing a new optical
interferometer capable of extremely
high resolution while peering through the Earth's blurry atmosphere,
U.S. Naval Observatory and
Naval Research Lab astronomers have been
able to detect the companion star to Mizar A.
This composite image of their observations shows the
daily and monthly relative orbital motion in the binary system.
Binary stars are a boon to astronomers because these stars can be weighed
-- their orbits providing a direct measurement of star masses.
APOD: 1999 November 20 - Small Star
Explanation:
A dim double star system cataloged as Gliese 623 lies
25 light-years from Earth, in
the constellation of Hercules.
The individual stars of this
binary system were distinguished for the first time
when the Hubble Space Telescope's Faint Object Camera
recorded this image in June 1994.
They are separated by 200 million miles -
about twice the Earth/Sun distance.
On the right, the fainter Gliese 623b
is 60,000 times less luminous than the Sun
and approximately 10 times less massive.
The fuzzy rings around its brighter companion, Gliese 623a, are image
artifacts.
The lowest mass
stars are classified as
red dwarf stars,
but even red dwarfs are massive enough to trigger
hydrogen fusion in their cores to sustain
their feeble starlight.
Slightly less massive objects, known as
brown dwarfs, can
shine only briefly as
their central temperatures are too low to utilize hydrogen
as nuclear fuel.
The present estimates of the mass of Gliese 623b are right at
this red dwarf/brown dwarf border but future observations should
help clarify the nature of one of our Galaxy's small stars.
Dim and difficult to detect, an
abundance of objects like Gl623b has been proposed as a possible
solution to the mystery of
"Dark Matter" in the Universe.
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.