What high-speed astronomy can tell us about the galactic zoo

Aeon ^ | Christopher Kochanek

[LibWhacker]

For most of human history, the distant ‘celestial sphere’ was regarded as perfect and unchanging. Stars remained in place, planets moved predictably, and the few rogue comets were viewed as atmospheric phenomena. This began to change with the Danish astronomer Tycho Brahe’s observation of the supernova of 1572 – apparently, a new star – and his studies of the Great Comet of 1577, which he proved was actually a distant object. Nonetheless, the impression of permanence is strong. There are very few astronomical objects that noticeably vary to the naked eye: only the brightest comets, novae and supernovae. For observers in the northern hemisphere, the last naked-eye supernova was in 1604.

Modern telescopic studies tell a quite different story. Today, we know of roughly a half-million variable stars in our galaxy, and identify thousands of transient objects each year. Although many stars vary in predictable ways, the Universe is also full of unpredictable violence. When two stars orbit close to each other, mass can flow from one to the other. If one of the stars is an old, collapsed white dwarf, the gas it pulls from its companion can accumulate until the dwarf undergoes a sudden thermonuclear explosion – a supernova like the one seen by Tycho. There is also another, more common type of supernova produced by the deaths of solitary stars more than about 10 times the mass of the Sun.

Supernovae show a broad range of behaviours that depend on the detailed properties of the system at the time of the final, fatal cataclysm. The atoms that emerge from supernova explosions have provided the raw material for all planets, including our own. Astronomers are understandably eager to learn more about them, but the two classes of supernovae combined happen only about once per century in our galaxy.

Obviously, for events occurring on time scales of a century, searching for them in our galaxy alone is not terribly profitable. Fortunately, our galaxy is only one of about a trillion galaxies in the visible Universe. If you monitor millions of galaxies all the time, it is possible to find many supernovae each and every day. This is one of the most exciting challenges of modern high-speed astronomy.

Other than supernovae, there are only a few variable sources luminous enough to be seen at the great distances to other galaxies, even using powerful telescopes. By far the most common is the random variability of quasars. Quasars consist of a supermassive black hole, millions to billions of times the mass of our Sun, which shine as material falls towards the black hole, heats up and radiates energy.

Today we think that essentially every galaxy contains a supermassive black hole at its centre, and something like 1 per cent of them are accreting mass fast enough to be seen as luminous quasars. The supermassive black hole at the centre of our own galaxy is essentially ‘off’. On rare occasions, though, such a black hole rapidly turns itself ‘on’. The most fascinating cause is a so-called ‘tidal disruption event’ in which a star like the Sun passes too close to the black hole and is ripped apart by the black hole’s tides. Some of the debris then falls into the black hole to power a transient flare. These tidal disruption events are far rarer than supernovae, occurring only about once every 10,000 years in any particular galaxy. In the distant Universe, the study of variability is essentially the study of black holes and supernovae.

This gives you some sense of the remarkable astronomical zoo of variable and transient objects. The challenge for the professional astronomer is to find and characterise all these different sources not only for how they work individually, but also to determine their overall demographics and statistics. To find large numbers of them, you need a big telescope that can detect the much more numerous distant, faint objects. In general, however, bigger telescopes see only smaller pieces of the sky. This frustrating rule can be bent only by spending large sums of money.

If your scientific goal is to find the largest possible number of transients, and to study their evolution across the cosmic history of the Universe, then you want to use a big telescope that covers as much of the sky as you can afford. This is fundamentally the goal of the Large Synoptic Survey Telescope (LSST). Located in Chile, LSST is (effectively) a 6.7-metre diameter telescope, scheduled to start full science operations in 2022.

LSST will be the closest astronomers have ever come to creating a movie camera to watch the whole universe. It will survey approximately half the sky using a camera that spans more than 40 times the area of the full Moon. But LSST can obtain a new image of each patch of that sky only once every three nights. LSST can detect transients 30 million times fainter than visible to the naked eye, making it a phenomenal project for finding huge numbers of faint transient sources across the visible Universe – LSST should find some 1,000 supernovae per day! But this capability comes at a cost: roughly $600 million just for construction, plus a significant operation cost as well.

At the other limit from LSST is a project I am working on: the All-Sky Automated Survey for Supernovae (ASAS-SN). By the end of this year, ASAS-SN will consist of 20 14-cm aperture telescopes spread across the globe, and costing roughly $3.5 million for both construction and operation through to 2022. With such small telescopes – big telephoto camera lenses, really – ASAS-SN can find only bright transients, roughly 25,000 times fainter than are visible to the human eye. Even so, it should still find about one supernova a day. And because ASAS-SN is comprised of small telescopes, it can image the sky far faster than LSST. The combined ‘image’ from all the ASAS-SN telescopes spans 1,600 times the area of the Moon. This allows them to survey the entire visible sky every night.

The two projects are highly complementary, essentially balancing a trade-off between ‘quantity’ and ‘quality’. LSST provides ‘quantity’: the large numbers of faint sources needed for statistical studies of distant sources, and for studying the evolution of transient sources across cosmic time. However, the typical LSST transient is faint and hard to study in detail for long periods of time, even with the world’s largest telescopes. ASAS-SN provides ‘quality’. The bright sources found by ASAS-SN are the ones that best survey the nearby Universe, and that can be studied in the greatest detail and for the longest periods of time using larger telescopes.

One of the most important tools for astronomers is the spectrum of an object: how much light is emitted as a function of its colour. A spectrum is the best way to classify the velocities, temperatures, elemental composition and type of an object (eg, which type of supernova? What were its unique properties?). Because you must chop up the light into narrow bins of colour, you need far more light to make a spectrum of an object than to get an image of it. LSST is already a large telescope, so it will be difficult or impossible to get a spectrum of the typical, faint LSST transient.

Even for the minority of LSST sources bright enough to obtain one spectrum, the source will quickly fade and become too faint to get another spectrum to study how it evolves with time. Therefore, a negligible fraction of LSST discoveries will be studied by this fundamentally important astronomical tool. The ASAS-SN transients are far fewer in number but are far brighter, so a very large fraction of ASAS-SN transients can be studied spectroscopically, and they can be studied for long periods of time even as they fade away.

Projects like LSST and ASAS-SN are continuing the revolution begun by Tycho, revealing the variable and sometimes violent events that light up the highly imperfect, ever-changing celestial sphere.

[dp0622]

Are there really other FReepers who understand this? :)

I would imagine some bright folks either work in the field or read a lot.

I’m still struggling through entanglement and how two atoms seven miles apart can affect each other faster than the speed of light.

[Telepathic Intruder]

"If you think you know quantum mechanics, you don't know quantum mechanics" -- Richard Feynman.

But this is an astronomy subject, involving classical physics, not quantum physics. When we tend to see the night sky as unchanging, forever constant, because nothing really does change. The stars are in the exact same positions they were a thousand years ago. The plants move with precise, clock-like regularity. But upon closer examination, we notice subtle changes. Supernovas, for example, occurring once every few dozen lifetimes. Last week's solar eclipse. A comet here, a particularly large meteor there. But upon even closer examination, the universe is full of change. Changes so violent they could vaporize our tiny planet with the merest exposure. But given the vastness of space, they are still few and far between.

[MadMax, the Grinning Reaper]

Gotta brag about what I saw Friday night. I was coming out of a 7-11 store at 9:14 PM when I saw a strange light slowly crossing from East to North (in Arlington, Virginia).

At first I thought it was an airplane slowing coming out of Reagan National Airport, and then that an engine might have been on fire because of the colors involved. Having seen hundreds of meteors in my lifetime, including one near Bolide and a very long-lasting Cherry colored one, I then realized what I was watching was a slow moving, relatively large meteor that has tear-dropped shaped (head swollen with a long tapered tail or like a large tadpole (large head, medium body, tapered tail).

It was a slightly dark Cherry red on the upper third, orange-yellow in the middle and white at the bottom third. It literally did a slow flight at about 20-30 degrees elevation from the ground and the whole thing lasted about 5 - 7 seconds, but nor more than 10. (The average meteor lasts about 1 second such as the Orange Perseid I saw last week about Aug. 18th).

This was the most spectacularly colored meteor I’ve ever seen (the long-lasting Cherry one from about 8 years ago was the first Cherry colored one I had every seen).

While I’ve always wanted to see a large flaming meteor , this one was very spectacular in its own right. (I missed a flamer by seconds during a Leonid shower about 10 years ago. My son and I were talking on the phone and we both saw one large meteor but I turned my head for a couple seconds and missed the sparkler one, to my chagrin).

Mother Nature sure knows how to put on a show. Next good one should be the Leonids in November. Remember “Watch the Skies”.

[dp0622]

Does the sun rotate? I dont mind asking stupid questions? :)

Do all stars rotate. All stars are made of the same stuff i would guess.

And that’s a complete guess :)

[LibWhacker]

Yes, but you’ve got to remember it’s a big ball of gas. So different latitudinal regions rotate at different speeds. And the interior rotates at different speeds than the outer layers. It’s just a big messy ball of hot churning gas, very different from a solid rotating ball.

[oldplayer]

Linebacker, thanks for an interesting post.

[dp0622]

That’s incredible.

I never cease to be shocked and surprised by science. By the universe.

[dp0622]

Fascinating stuff.

[Lazamataz]

I hold that the universe is much too large, and by it’s volume, is a white-privilege racist oppressor. Let’s burn a statue of Galileo.

[LibWhacker]

LOLOL

[Mr. K]

the sun rotates every 11 days

[TXnMA]

Thanks for a good read! It's encouraging to know that the concept of large-baseline, distributed small telescope networks is being exploited.

I'm ending my run of solar orbits, but, maybe one day, there will be an "internet of amateur telescopes" whose shared and compiled data amplifies the spatial and time and spectral resolution of ASAS-SN...

[Alas Babylon!]

This guy makes a good case for the ASAS-SN, which is the project he’s working on, and damns with faint praise the LSST.

I wonder if there’s a fight for grant money?

[Techster]

Rotation at equator 25.05 earth days
Rotation at 16 degree latitude 25.38 earth days
Rotation at poles 34.4 earth days

Wikipedia https://en.wikipedia.org/wiki/Sun

[2nd Amendment]

Lots of things are racist! For instance the solar eclipse last week skipped predominantly black cities and went to predominantly white cities!

[dp0622]

Good Lord!! It must be YUGE!

Do all the planets and the sun rotate at the same speed?

[LibWhacker]

Also...

The sun's core rotates four times faster than its surface

[Telepathic Intruder]

Just about everything in space rotates to at least a little. The sun’s rotation is differential; it takes 24 days at the equator and 33 days at the poles. The sun itself, and all stars, are made of about 75% hydrogen and 25% helium, the main elements of the universe. The earth and everything else we know are mostly just trace heavy elements, created by the explosion of some star 5 billion years ago.

[Wuli]

I would like to see automated astronomical observatories put together on and run remotely on earth’s moon, where NOTHING way out in space is as obstructed as everything is by earth’s atmosphere.

I have even wondered about saving the Hubble telescope, moving it and keeping it operating in lunar orbit.

I imagine in a couple hundred years we may even do some such thing far out on or around Pluto.

I can even envision, closer to home, permanent earth-sent satellites orbiting every planet in the solar system, monitoring every observable change and with analysis of all the data looking for, and I believe seeing, solar system wide relationships to some events and statuses of what is observed individually on ours and the other planets.