Black hole.

Psych, that's not what it is.

One of the most famous black holes is 1/2 of Cygnus X-1 , a binary system in which only one companion emits visible light.





Astronomers have long hypothesized that the "dark star" in this binary was a black hole, but true proof did not come until last week, with the publication of three papers in the Astrophysical Journal--one precisely measuring the distance, one precisely measuring the mass, and one precisely measuring the spin.





The Original Question

Does the massive blue star (which we can see, as shown by my use of the word "blue") orbit a black hole or a neutron star?









Why Astronomers Couldn't Tell

Neither black holes nor neutron stars emit light in the way stars traditionally do--by energy released during fusion. Both black holes and neutron stars are very massive, a dying star requiring only slightly more mass to collapse into a black hole than to compress into a neu tron star. A small difference or imprecision could mean the difference between this object being the most extreme type of star and the second most extreme type of star, and you know how upset stars get when you get their extremity wrong. The precise distance from Earth to the Cygnus X-1 system was unknown.





What Astronomers Don't Know

Astronomers did know that the two objects orbited each other every 5.6 days, and they could have used that orbital period to calculate (using equations you learned in high school physics and totally could still write down on command) the mass of the unidentified dark object (UDO).





However, this calculation would require knowing the mass of the blue star.





Well, that's easy, right? Astronomers find out stars' masses all the tiiiiime. In fact, that must be at least 54% of their jobs. No prob. Give the project to a summer undergrad intern.









What We Know How to Do

If we know the distance to a system, we can calculate how much the light dims while traveling that distance, and we can then go back and figure out how much light it would be putting out if you were standing right next to it with a light-measurer.









Why That Matters for Cygnus X-1

Since the distance to Cygnus X-1 is unknown, we can't figure out how much it dimmed due to travel across space. Since we don't know how much it dimmed due to travel across space, we don't know how bright it would be if we were hanging out near the corona (yeah, it means more than beach beer).





Let's say, for demonstration's sake, that we think the distance to Cygnus X-1 is 2 miles. However, the distance is actually 5 miles. If we put "2 miles" into the magical equations, we will underestimate how bright the blue star is (because we will think less of its light was lost in transit). If we, instead, thought Cygnus X-1 was 10 miles away, we would overestimate how bright the blue star is, as we would think a larger portion of its light output didn't reach us.





Underestimating the brightness of the blue star leads to underestimating its mass. Underestimating its mass leads to underestimating the mass of the companion star (the black-white-hole-neutron star), because it then takes less gravity to account for the motions we observe.





So What Ha-Happened?

Propaganda-style graphic for the VLBA. Source

VLBA , a project with which the NRAO is involved; go, employer) to observe this system. Although the "dark star" (which sounds like some kind of superstitious mumbo-jumbo) emits no optical waves, it does emit radio waves, like all truly cool astronomical objects. Reid, et al., the authors of the three aforementioned papers, used an instrument called the Very Long Baseline Array (the, a project with which the NRAO is involved; go, employer) to observe this system.





The VLBA is an interferometer consisting of ten (25-m diameter) radio telescopes located all over the world. When data from these ten telescopes are combined, the telescopes act as one single telescope that is the size of the largest separation between any two telescopes. So, for instance, if you had ten telescopes in your backyard, and the ones in completely opposite corners were 50 feet apart, you a) need a different backyard and b) have a 50-foot diameter telescope (provided you have written baseline-analysis software, which I know you have).





The VLBA can make maps that have 100 times the angular resolution of the Hubble Telescope's maps.





Yeah, it's from Wikipedia. Reid, et al., used the VLBA to measure parallax , or how much an object's position changes relative to the positions of farther away, background objects. Think of a driving in a car: the trash on the side of the road passes by you much faster than the mountains on the horizon, so if you wanted to know something about the trash's distance from you, you could look at its position relative to the background mountains from two different locations.





Parallax says: Cygnus X-1 is 6,050 +/- 400 light-years from Earth. While that may, to those of you who aren't used to astronomical (double-entendre) errors, seem like a large error bar, it's enough to tell whether the "dark star" passes the threshold for being a black hole. A 400 ly uncertainty gives a mass uncertainty of 1 solar mass, and even if you subtracted 1 solar mass from the dark star, it would still be a black hole.









What I Think is Interesting about This Discovery

Even though Stephen Hawking conceded that this was not a neutron star twenty years ago, no one had actually proven him wrong until last week. Science sometimes moves slowly, since conclusions do have to be solidly proven, even when you "know" they are true. Determining distances to objects, and masses of objects, is actually hard! These calculations seem fundamental. After all, scientists are figuring out what kind of exotic matter might or might not be inside neutron stars; scientists can create miniature black holes on Earth. How could they not figure out how far away something is? But these calculations are not trivial, nor are they certain. All information relies on other information. We couldn't know the nature of Cygnus X-1 until we knew the mass of the dark star, and we couldn't know the mass of the dark star until we knew the mass of the bright star, and we couldn't know the mass of the bright star until we knew how bright it was, and we couldn't know how bright it was until we knew how far away it was, and we couldn't know how far away it was until we measured its relative movement using a telescope the size of a continent. WHAT? Yes. In astronomy, there is no "lab." You can't set up conditions and then test them. You have to take what you're given and do what you can with it. You have to wish Cygnus X-1 were closer so parallax could be measured more easily, but that will never happen because that is magic, so then you just have to get better at measuring parallax.









Astropyhsical Journal arXiv: Lijun Gou, Jeffrey E. McClintock, Mark J. Reid, Jerome A. Orosz, James F. Steiner, Ramesh Narayan, Jingen Xiang, Ronald A. Remillard, Keith A. Arnaud, & Shane W. Davis (2011). The Extreme Spin of the Black Hole in Cygnus X-1arXiv: 1106.3690v1





Astropyhsical Journal arXiv: Jerome A. Orosz, Jeffrey E. McClintock, Jason P. Aufdenberg, Ronald A. Remillard, Mark J. Reid, Ramesh Narayan, & Lijun Gou (2011). The Mass of the Black Hole in Cygnus X-1arXiv: 1106.3689v1



