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: n alone (the remnants of single stars), or n 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) is not giving off any detectable light, but (b) 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: n n n 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. Search for X-ray sources in the sky, and determine their precise positions. 2. Monitor the motion of any star found at exactly that location. Is it orbiting around an unseen companion? 3. 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. 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. 2. Some stars vary in brightness anyway for example, eclipsing binaries and pulsating stars. How do we discriminate? 3. 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.