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Finding Black Holes
Left Behind by Single Stars
Finding Black Holes
"Yesterday upon the stair
I met a man who wasn't there.
He wasn't there again today.
I wish that man would go away."
Hughes Mearns (1875-1965)
Three Interesting Mass Ranges
1.
Supermassive black holes, formed by
the coalescence of myriads of stars.
We have already discussed this and described a local
example, in the centre of the Milky Way!
At the Other End:
2.
Mini black holes , ‘left over’ from the early
days of the universe, when everything was
more densely packed. Black holes of this sort
would have a mass like that of an asteroid or a
mountain, and could not be routinely created in
the present-day universe.
We defer this discussion to the next module.
Here, We Will Focus On:
3. Stellar mass black holes – those left
behind after a massive star undergoes a
supernova.
They could be:


alone (the remnants of single stars), or
in a binary pair, with a normal star as a
companion
We Start with
Black Holes in Binary Stars
The strategy: find a binary star in which a visible
member is clearly in motion, orbiting around a
companion that:
(a)
(b)
is not giving off any detectable light, but
is clearly too massive to be a dim white dwarf
or a neutron star
One Serious Problem:
Such Leftovers Will Be Rare!
Only the most massive stars produce supernovae
and black holes.
But such stars are rare! - for every O star, there
are millions of faint red dwarf stars
So: most pairs of binary pairs will not contain a
black hole!
The Tell-tale Evidence
The spectrum of the visible star will reveal that the
star is moving regularly, towards and away from
us. (We use the Doppler shift.)
But we need to observe each target many times, to
monitor for and discover changing velocities.
That takes time and effort, even for just one target.
Which Stars are Promising
Candidates?Maybe 1 or 2 of these, say… but which
ones?
Let’s Narrow the Field
Think back to novae. Such objects consist of a binary pair,
one of which is a a white dwarf.
Now remember the effects of mass transfer from the
expanding normal star to the white dwarf:



an accretion disk forms
the impact of infalling gas as it meets the disk causes
extreme heating
and thus the emission of X-rays
The same should happen here!
Like So!
(note the tiny black hole in the center of the
accretion disk, and the ‘hot spot’ at lower left)
So: A Logical Search Strategy
1.
2.
3.
Search for X-ray sources in the sky, and determine
their precise positions.
Monitor the motion of any star found at exactly
that location. Is it orbiting around an unseen
companion?
If so, use Newton’s laws to deduce the mass of
the dark companion. Perhaps it’s a black hole?!
The Constellation
of Cygnus
Success!
Cyg X-1 is an X-ray source associated with a bright blue B
star.
In 1973, it was shown to be in a binary system with a
massive dark companion – a black hole! [This work was
done at the David Dunlap Observatory, in Richmond Hill,
just north of Toronto when I was a grad student there.]
Many other examples are now known.
Sometimes We Even See Eclipses
[the X-rays disappear when the black hole goes behind the star]
So Much for Binaries –
Can We Find Isolated Black Holes?
How should we proceed? Make your guess!
Are These Good Black Hole Candidates?
I’ll Bet You Got it Wrong!
The ‘obvious answer’ is wrong!
It will not work simply to look for spots of
extreme darkness in space, as you might
naively have thought.
Why Don’t the Black Holes Look Dark?
Remember that the black holes don’t ‘suck in’ light from a
surrounding area and create a dark mark like an inkblot.
Paradoxically, we can find invisible black holes by looking
for the enhanced brightness of background stars.
In short, let’s identify stars that look brighter than
expected.
The rationale is as follows: this could be caused by
gravitational lensing if a dense lump in the
foreground (perhaps a black hole?) lies between us and
the star.
Okay, But Which Star?
Does one of these
look‘brighter than
it should’? (Maybe
the one just above
the centre?)
But perhaps that star
just happens to be
closer than most
of the others!!
Fortunately, Stars Move Through Space
(and so do the Black Hole remnants
left behind after their deaths!)
If a black hole drifts between us and a distant star,
we will see a temporary brightening of the star
and then a return to dimmer levels. This draws
our attention to the phenomenon.
[This is probably exactly the opposite of what you
expected. In some ways, it is the inverse of an eclipse!]
There are Challenges
1.
2.
3.
Black holes are rare, and will be only briefly lined up
with any particular star. We need to study millions of
stars for many years if we hope to catch even a few
examples of this temporary brightening.
Some stars vary in brightness anyway – for example,
eclipsing binaries and pulsating stars. How do we
discriminate?
Any single event will never be repeated, so we can
only work out statistical estimates of black hole
masses and numbers.
The MACHO Project
MACHOs are MAssive Compact Halo Objects
[They are called ‘Halo’objects because we search for them
in the outer parts – the ‘halo’ – of our galaxy]
As Time Passes:
1. Dim
2. Bright
3. Dim
A Practical Problem:
How Do We Study
Millions of Stars at Once?
1. Find a collection of many stars at some moderately
large distance – like a nearby galaxy – so they can
all be captured in a single big image.
2. Then take picture after picture, day after day, week
after week, year after year, and look for short-lived
changes in brightness.
3. Finally, automate the whole process!
One Very Helpful Thing
The colour of a star does not change when it is seen
through a gravitational lens. [This is because all light
behaves the same way under gravity!]
By contrast, pulsating stars undergo temperature (and thus
colour) changes.
So we can distinguish ‘lensing events’ from actual variable
stars.
The MACHO Project
[monitor the stars in the Large Magellanic Cloud]
http://wwwmacho.anu.edu.au/
A MACHO Event Observed
How It
Behaved
What We Have Learned
In the MACHO project, astronomers monitored the
behviour of about 12 million stars over the span of a
decade or so.
About a dozen certain microlensing events were detected
(plus lots of previously unknown variable stars, a rich byproduct).
This allowed the astronomers involved to set some
interesting limits on the numbers of black holes in the halo
of our galaxy.