Stars are usually classified by observing their color in visible light,
since this is the portion of the spectrum where they are usually the brightest.
We've already learned that accretion disks around
black hole will also emit some visible light.
This means that if we want to view the companion star of the black hole,
we'll have to wait until a black hole was finished eating a major meal
so that the disk isn't emitting light which would otherwise pollute our image.
When astronomers want to classify a star,
they look at it using different filters to determine the star's properties.
Low-mass stars with masses less than the Sun's mass are dim
and have colors that range from yellow to orange to red.
High-mass stars are bright and are blue in color.
Since low-mass stars are dim,
they can be difficult to detect.
So, sometimes we have trouble detecting the companion in a low-mass X-ray binary,
and the binary is classified based on it's X-ray emission instead.
The companion stars in
high-mass X-ray binary systems are usually easier to see since they are so bright,
meaning that in many cases,
we can also obtain detailed spectrum of the star.
So, it isn't at all surprising that the first confirmed black hole,
Cygnus X-1 has a bright blue high-mass companion star.
However, accretion disks can also look very blue,
bluer in fact than hot blue stars.
This means that when the disk is bright,
it can be incredibly hard to work out what kind of star is feeding the compact object.
X-ray images of black holes are not quite as impressive
to look at as some of the other types of images we've seen in this course.
They can be fairly featureless with
just a series of dots scattered in a black section of the sky,
except, of course, when they're suddenly change.
The left-hand image shows an X-ray image of the sky near our old friend, Cygnus X-1.
In the left image taken before June 2015,
we see full bright, X-ray point sources.
Cygnus X-1 is the brightest X-ray source in Cygnus and a high-mass X-ray binary.
Cygnus X-3 was the third X-ray source
discovered in Cygnus and is a low-mass X-ray binary.
At this moment, it is unknown whether there is
a neutron star or a black hole in Cygnus X-3.
3A 1954+319 is also a low-mass X-ray binary,
most-likely harboring a neutron star.
Cygnus A is a supermassive black hole,
but it looks dim because it's in a galaxy far,
far away, while the other sources are in our own galaxy.
A small X marks the spot where the low-mass X-ray binary, V404 Cyg,
suddenly became as bright as Cygnus X-1 and Cygnus X-2 in June 2015.
V404 Cyg is close to 8,000 light-years away from us.
The companion is a type K star,
which means that it's orange in color and has
a mass that is just 40 percent of our Sun's mass.
The black hole has a mass that is seven times our Sun's mass.
So, there is no danger that this could be a neutron star masquerading as a black hole.
In this movie, the black hole and it's
accretion disk are the bluish white light at the center of the image.
The accretion disk suddenly erupted on June 26th,
2015 emitting X-rays in all directions.
These X-rays form a spherical shell front that
expands and collides with dust clouds far away from the black hole.
The red rings are X-rays that are reflected
off the dust that lies between the black hole and the Earth.
Although the wave front is a sphere,
we see circles since the dust clouds are a series of
surfaces between the black hole and the Earth.