What is a black hole? Do they really exist? How do they form? How are they related to stars? What would happen if you fell into one? How do you see a black hole if they emit no light? What’s the difference between a black hole and a really dark star? Could a particle accelerator create a black hole? Can a black hole also be a worm hole or a time machine? In Astro 101: Black Holes, you will explore the concepts behind black holes. Using the theme of black holes, you will learn the basic ideas of astronomy, relativity, and quantum physics. After completing this course, you will be able to: • Describe the essential properties of black holes. • Explain recent black hole research using plain language and appropriate analogies. • Compare black holes in popular culture to modern physics to distinguish science fact from science fiction. • Describe the application of fundamental physical concepts including gravity, special and general relativity, and quantum mechanics to reported scientific observations. • Recognize different types of stars and distinguish which stars can potentially become black holes. • Differentiate types of black holes and classify each type as observed or theoretical. • Characterize formation theories associated with each type of black hole. • Identify different ways of detecting black holes, and appropriate technologies associated with each detection method. • Summarize the puzzles facing black hole researchers in modern science.
09.03 – Classifying Black Hole Binaries by their Food Source
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From the course by University of Alberta
Astro 101: Black Holes
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From the lesson
Our Eyes in the Skies
Black holes change over time. This module will focus on how and why black holes change as well as how we look for these changes.
Meet the Instructors
Sharon MorsinkAssociate Professor
Department of Physics, University of Alberta
Stellar-mass black holes are
most easily identified when they are accompanied by a companion.
The gravitational effect of a black hole on
its companion star can help give us a location where the black hole might be hiding.
If the black hole is actively feeding on
the companion star we will be able to see
this clearly in the X-ray portion of the spectrum.
In fact, because these systems are so bright in X-rays,
they're often referred to as X-ray binaries.
We should note however that this term refers to
systems containing a star and a compact object.
As we're aware compact objects can be either neutron stars or black holes.
As such, it is important that when we observe
these systems we try and find the mass of the compact object.
If the mass of the compact object is more than three solar masses,
it must be a black hole.
If it's lighter than that,
it's likely a neutron star.
In some cases though it can be very hard to tell the mass of the compact object.
When astronomers are unsure of the characteristics of systems
like this they list them as a black hole candidate.
There are two types of X-ray binaries:
high mass X-ray binaries and low mass X-ray binaries.
But this classification is not based on the mass of the compact object.
It may seem strange to you at first but X-ray binaries are
classified by the mass of the companion star not the compact object.
The companion stars in low mass X-ray binaries
have masses that are the mass of the sun or smaller.
High mass X-ray binaries have companion stars
that are at least ten times more massive than the sun.
Anytime astronomers come up with a classification like this,
you'll find that some objects don't quite fit.
So we also have an in-between group
that is sometimes called intermediate mass X-ray binaries.
But their properties are usually pretty similar to low mass X-ray binary group.
Why would astronomers choose to classify binary systems based
on the type of companion star rather than the type of compact object?
The reason for this classification is that the properties of
the system depend more on the type of donor star than the type of compact object.
What this means is that observations of these systems vary more
dramatically if you compare high mass and low mass X-ray binaries,
than if you were to compare
stellar mass black holes and neutron stars that are both feeding on,
say, a low mass star.
When then companion is a low mass star such as in a low mass X-ray binary,
the gas from the companion star flows to the black hole via
Roche Lobe overflow that we studied in an earlier module.
Also, recall the high-mass stars tend to have
larger outfalls of material in the form of powerful stellar winds.
In high mass X-ray binaries,
the mass loss through wind ends up being accreted onto the black hole,
this is called wind fed accretion.
However, we should note that high mass stars can
also feed black holes via Roche Lobe overflow.
Typically, low mass companions are small in size,
while high mass companions are large.
Small stars can orbit closer to the black hole than large stars can.
Kepler's laws of motion tells us that stars with small orbital separations,
orbit with faster speeds and take shorter amount of time to orbit.
Low mass X-ray binaries typically have
short orbital periods that can range from less than an hour to many hours.
Meanwhile, the larger companions in high mass X-ray binaries orbit further away
from the center of mass of the system and can take a few days to complete one orbit.
This means that the feeding or mass transfer mechanism and so the rate of
mass transfer along with
the orbital period can be greatly impacted by the type of companion star.
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 is 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 its 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 disc 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 and 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 this suddenly change.
The left hand image shows an X-ray image of the sky near her 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 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 plus 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 8000 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
its 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 wavefront is a sphere,
we see circles since the dust clouds are a series of
services between the black hole and the earth.
Another example of a low mass X-ray binary is
the X-ray source X9 in the globular cluster named 47 TUC.
A globular cluster is a dense star cluster that can have many millions of stars.
Since the stars are closer to each other than in part of the galaxy where we live,
the stars can easily hook up with other stars to
form binary systems through dynamical formation.
So, if you were to randomly choose a globular cluster to look at with an X-ray telescope,
you'd have a good chance of finding an X-ray binary.
The X-ray binary X9 is still classified as
a candidate black hole since its mass has not yet been measured.
But the orbital period is very small,
only 25 minutes and the companion star is most likely a white dwarf.
The companion star to the black hole Cygnus X-1 is easily seen as
the bright star in the very center of
this visible light image of the constellation Cygnus.
Since this is a visible light image,
we can't see the accretion disk of the black hole.
The red light is coming from glowing hydrogen gas in a nearby star-forming region.
Cygnus X-1's companion star is named HDE 22 6868.
But for obvious reasons,
we normally call it Cygnus X-1's companion star.
The companion is a type or supergiant that has a larger mass than the black hole.
The companion star's mass is 19 times larger than the sun,
while the black hole's mass is 15 times the sun's mass.
The two objects orbit their common center of mass which is closer
to the companion once every 5.6 days.
Both high mass and low mass X-ray binaries are spotted scattered throughout galaxies.
They are relatively easy to spot because the black holes have
their dinner sitting right there next to them in the binary system.
What happens when we switch up to other size scales?
What are the alternative diets for supermassive black holes?
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