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May212013

SS Cygni: A red dwarf-white dwarf pair that's closer than scientists thought (or, how Hubble got told)

Determining distances to objects in space is no easy task, unless you're some kind of special person with an impossible faster-than-light spaceship and can just measure the miles between Earth and UDFj-39546284. This is what it's like to be in a helicopter above SS Cygni, minus the arrows and text (Credit: J. Miller-Jones [ICRAR], using software created by R. Hynes).But no astronomer I know has such a ship, and so they all are likely to say, "That galaxy is 3 billion light-years away, plus or minus a million light-years."

And that level of uncertainty doesn't apply only to far-away objects. It also applies to relatively nearby stars, such as SS Cygni, a close binary system in which a red dwarf and a white dwarf orbit each other every 6.6 hours. The Hubble Space Telescope had pegged the distance between us and SS Cygni at 540 light-years.

And, well, it's not often that you get to tell the Hubble Space Telescope it's wrong. But when the Hubble Space Telescope is wrong, you gotsta tell it loud and clear. So, "Hubble, SS Cygni is more like 372 light-years away. Take that in your face."

And what instruments were able to so diss the high-res space telescope? The Very Long Baseline Array (VLBA), a combination of 10 radio antennas spread between Hawaii and St. Croix, giving the telescope as a whole an effective diameter of more than 5,000 miles, and the European VLBI Network (EVN), which has antennas in Europe, China, Russia, and South Africa.

Here's the low-down on the SS Cygni system, how scientists used the VLBA to determine its distance from us, and why that 168 light-year revision matters.

What is this binary system like?

The solar flare in the upper left extends 100,000 miles from the Sun's surface, the same distance as the separate between SS Cygni's two stellar components.

Tight. The red and white dwarfs are separated by only 100,000ish miles. If the Sun were that close to another star, the star would begin just at the end of the solar flare in the image to the right.

Such a close sharing of quarters can lead to violent outbursts on the part of the white dwarf. The white dwarf roommate siphons material--Ramen noodles, the last of the Fruity Pebbles, clean underwear--from the red dwarf roommate, and all that stuff forms a disk around the white dwarf. When the white dwarf is taking a lot of material, the relationship is stable (and the analogy totally unravels), but when the transfer rate is lower, the disk destabilizes and undergoes an outburst. 

This burst happens with regularity--in the case of SS Cygni, once every 49ish days. Because of these explosions, it is categorized as a dwarf nova. It's only during the burst period that the system emits radio waves. 

But it didn't appear to work the same way all the other dwarf novae do.

This is what astronomers thought until today: SS Cygni is too bright. It's so bright that lots of mass must be changing hands. So much mass that the disk should always be stable. Meaning SS Cygni should never, ever have an outburst. But it does--often. If SS Cygni is in fact 500 light-years away, and is super bright but does the nova thing anyway, our theories about how dwarf novae work must be wrong.

So when amateur astronomers from the American Association of Variable Star Observers (AAVSO) noticed that it was, in fact, having an outburst, they informed professional astronomers, who perked up and took action because they were eager to prove that their theories were not, in fact, wrong.

When an object emits radio waves, and you have a 5,000-mile diameter radio telescope, you can do science. 

Who are these so called "professional astronomers," and what did they do with that gigantic telescope?

A team led by James Miller-Jones from the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR) took advantage of the radio emission to watch how it "wobbled" compared to the galaxies behind it (from our perspective).

"The wobble we were detecting is the equivalent of trying to see someone stand up in New York from as far as away as Sydney," Miller-Jones said in a press release.

Why couldn't astronomers do that any old time using the optical light, even if it's not emitting radio waves?

After all, SS Cygni is always putting out visible light.

"Our key advantage was using radio telescopes to observe the system. In visible light, optical telescopes like Hubble see hundreds of different stars, all of which are moving by different amounts, whereas in radio waves the background we compare against is much further away and therefore doesn't appear to move at all," co-author Assistant Professor Gregory Sivakoff from the University of Alberta, said.

Energetic radio galaxies were much more common at the beginning of the universe, so most are extremely far away.

Why are far-off things better for determining distance?

Astronomers were trying to find SS Cygni's parallax, which is the numerical measurement of the "wobble"--how much it appears to move relative to a distant background when the observer's position changes.

When you're looking out the passenger window of a car, the road's shoulder is flying by you, but the mountain peak moves slowly across your view. If we can measure the angle by which an object appears to move due to our motion, we can determine how far away it is--this is parallax.Parallax is based on the concept that objects close to you seem to "move" faster when you're moving than far-away objects do. I'm about to plagiarize myself here, but I'm being honest about it. "When you're looking out the passenger window of a car, the road's shoulder is flying by you, but the mountain peak moves slowly across your view." Because of that differential, astronomers can measure the difference between how the "shoulder"and the "mountain peak" change relative to each other and thus determine how far away the "shoulder" is.

Wikipedia user Natejunk2004, aka Best Screen-Named Person Ever, made this animation demonstrating what I clumsily tried to convey above.

Miller-Jones, et al., looked at SS Cygni when Earth was at different, widely separated points in its orbit (see the figure to the left) and observed where it was relative to background radio galaxies from each of those perspectives. The radio galaxies, by the way, are so distant that they don't seem to move at all.

So they used parallax to figure out that SS Cygni is 168 light-years closer than they'd previously thought. So what?

Well, if SS Cygni is closer to us, it's not quite as intrinsically bright as scientists thought. It's just nearby (in the same way that the Sun seems to be really bright compared to Sirius but is really just average and next-door). Which means that the mass transfer rate is low enough to create an unstable disk. Which means that astronomers were right about dwarf novae (or, at least, that they weren't wrong in this particular way).

And we all know how astronomers love to be right. Right?

But, really, using evidence to prove that a physical process does mesh with a theory is truly a significant scientific feat. It is, in fact, what science is all about.

 

 

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