Copyright © Michael Richmond.
This work is licensed under a Creative Commons License.
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Those crazy astronomers! If you listen to one of their conversations, you'll find that it is sprinkled with -- no, chock full of -- strange terms and phrases that make no sense. If you intercept their E-mail messages, you'll see what appears to be a secret code: MCG +01-3-34, J053432+220051, 2012 DA14. What's going on?
Don't worry. It's not a secret plot to take over the world; it's just the native lingo. Astronomers have typically spent five or ten or twenty years working in a specialized field, studying a small subset of objects in the sky, writing and reading papers which discuss these objects in minute detail. Because we -- like most people -- are LAZY, we have come up with a number of ways to use a few words or characters to express complex ideas. For example, instead of writing
the big spiral galaxy just north of a spot between the two stars at the end of the handle in the Big Dipper
we prefer to write
M81
It saves 108 characters. If you have to mention this galaxy five or six times in the course of a lecture, this can really add up!
Astronomers aren't really any different from other specialists. For example, here's a small sample of a recent posting from Sauropod Vertebra Picture of the Week
But the ventral body profile still had to meet the distal ends of the pubes and ischia, which really can't go anywhere without disarticulating the ilia from the sacrum (and cranking the pubes down would only force the distal ends of the ilia up, even closer to the tail - the animal still had to run its digestive and urogenital pipes through there!).and here's one from a baseball website I often visit
331 / 417 / 559 3630 475 1599 BB / 696 K Just look at those numbers. God damn.
If astronomers are wierd, then paleontologists and baseball fans are wierd, too.
Of course, they all probably ARE kind of wierd, when you think about it.
It is of course impossible to cover all the strange terms which astronomers use in just one short lecture ... but I'll try to mention some of the most common ones, and give you a few tips so that you might be able to figure out some of the others yourself.
Astronomers tend not to use the ordinary units -- meters, kilograms, miles, pounds -- when discussing their favorite objects. Why not? Because astronomers prefer not to use scientific notation; we like it better when all the numbers lie in a small range, say, between 0.01 and 1000. For example, instead of writing the mass of the star Vega as 4.25 x 1030 kg, we prefer to write 2.14 solar masses. See -- isn't that easier?
2.14 vs. 4.25 x 1030
Keep that goal in mind as we walk through the garden of astronomical units.
But notice that we are not actually mentioning the astronomical unit.
Quick: how many days was it between these two events?
Athletics win World Series Oct 26, 1911
Titanic sinks April 15, 1912
----------------------------------------------
time between =
Not so easy, is it? Try again with Julian Dates:
Athletics win World Series 2,419,335
Titanic sinks 2,419,507
----------------------------------------------
time between =
But astronomers have a much broader span of time to cover -- the entire history of the universe. You'll hear them use these terms:
Q: What is the crossing time of Jupiter?
Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.
Q: Who wrote this?
the answer
As a result, astronomers have devised a set of units which are reasonably well matched to commonly encounted distances in space. Among them are the
Why don't astronomers use light-years? Well, the light-year is roughly the same size as the parsec, and if we used both, we'd have to remember how many light-years are in one parsec (and vice versa). Remembering is hard, so we don't bother.
Sometimes, astronomers describe the distance to an object in an indirect manner. There are good reasons for the following approaches, but it can be really frustrating at first glance.
For example, consider two Sun-like stars, which produce exactly the same amount of light, and hence the same absolute magnitude M = 4.34 . Alpha Centauri is our closest stellar neighbor, and so appears very bright in our sky; HD 6818, which is much farther away, appears too faint to see without a telescope.
| Name | Abs mag M | distance (pc) | App mag m | (m - M) |
| Alpha Cen | 4.34 | 1.34 | -0.01 | -4.35 |
| HD 6818 | 4.34 | 101.3 | 9.37 | +5.03 |
Most stars have masses between 0.01 and 100 solar masses, so this is a very convenient unit for individual stars.
Galaxies are made up of billions or trillions of stars, so one might expect astronomers to devise a unit like the "galactic mass" --- but we haven't. This is one case (see another below) in which astronomers do use scientific notation. When we write the mass of the Milky Way or the Andromeda Galaxy, we end up with something like 1012 solar masses.
In recent years, as our ability to detect smaller planets -- and the amount of $$ available to study them -- has increased, astronomers more and more often talk about Earth masses:
The Milky Way Galaxy, for example, has a luminosity of
roughly 200 billion
.
Why don't astronomers create a unit which represents the luminosity of an entire galaxy? If they did, they wouldn't have to keep including factors like 1011 when talking about galaxies, or 1013 when talking about quasars.
Why don't they? Beats me.
I do not think it means what you think it means
"Hot". "Cold". "Hard". "Soft."
Simple words, right? You may use them every day, and you may think you know what they mean. But if you were to hang out with astronomers, you'd soon hear them used in completely new and mystifying ways. Let's examine several very familiar words and phrases which astronomers have, um, bent to serve their own purposes.
In ordinary life, what does "hot" mean?
What does "cold" mean?
But in astronomical circles, these words have a slightly different meaning, one that describes the amount of disorder in the system.
Picture a spiral galaxy with a disk of gas and dust.
Image of NGC 5746 courtesy of Sherry and Steve
Bushey/Adam Block/NOAO/AURA/NSF
Focus on the disk. It consists of gas and dust and stars, arranged in a flattened circle, rotating around the center of the galaxy.
If the objects in the disk have individual motions which follow the rotation very closely -- in other words, if the individual objects have velocities which consist of very little vertical motion, and just enough horizontal motion to keep them in a circular orbit -- then we call the system "cold".
But if the objects in the disk have components of their velocities which differ significantly from a purely circular orbit -- in other words, significant motions perpendicular to to the disk, or radially inward and outward -- then we call the system "hot".
As you might imagine, a "cold" disk will be relatively thin, relative to its diameter,
while a "hot" disk will tend to be thicker: relative to its diameter,
In ordinary life, what does "hard" mean?
What does "soft" mean?
If you send light through a prism (or a diffraction grating), it breaks up into a spectrum.
Image courtesy of
CompareWeightLossProgram.com
You may be familiar with the appearance of optical spectra -- the relative amounts of visible light that stars emit, from blue through green to red.
We can display the spectrum of a star without colors by making a graph which shows the intensity of radiation at different wavelengths.
But astronomers have telescopes which can detect light far beyond the visible part of the spectrum. The XMM satellite, for example, measures X-rays from celestial objects.
Yes, X-ray astronomers place the highest energy photons on the RIGHT side of their graphs, whereas optical and infrared astronomers usually put the highest energy (blue) photons on the LEFT side of their graphs. Isn't science wonderful?
Click on the graph ...
Figure 3 from
Bernardini et al., A&A 498, 195 (2009) .
In this case, the pulsar's X-ray emission changed from "hard" to "soft" over a period of several years.
In ordinary life, what does "dropout" mean?
In recent years, astronomers have been looking very hard for a very particular type of "dropout" ..... "dropout galaxies". What does that mean?
Well, if you look at the light emitted by stars in a galaxy, you will find that most of it appears in the optical and infrared regions of the spectrum. It turns out that very, very little light is emitted in the far-ultraviolet.
If you look at a galaxy like this through the standard optical filters called "g" (which transmits blue light) or "i" (which transmits very red light), you'll see some photons -- and so, you'll see the galaxy.
But if the galaxy is very far away from the Milky Way, it will be carried away from us by the expansion of the universe. Because it is moving away from us, the Doppler Shift increases the wavelength of the light we receive from the galaxy. At a redshift of z = 1, the spectrum would look like this:
Q: Do we still see light from the galaxy when
we look through the "g" filter?
Q: Do we still see light from the galaxy when
we look through the "i" filter?
Yes and yes. Right.
If the galaxy is more distant, its light will be more shifted. Look at the situation if the redshift is z = 2.
Q: Do we still see light from the galaxy when
we look through the "g" filter?
Q: Do we still see light from the galaxy when
we look through the "i" filter?
Yes and yes. We still see the galaxy in both filters.
But if the galaxy is REALLY distant, say, at z = 6, then look at what happens:
Q: Do we still see light from the galaxy when
we look through the "g" filter?
Q: Do we still see light from the galaxy when
we look through the "i" filter?
No! There's very little light left at the wavelength of the "g" filter. If we look through that filter, we won't see anything in the sky at the location of the galaxy.
The galaxy will have dropped out of the "g" filter.
And that is exactly what astronomers see when they take very deep images with HST or other big telescopes: certain very distant galaxies do appear in the reddest filters, but do NOT show up in bluer filters. Look at this picture, for example:
Andrew Bunker and his co-authors identify 12 sources which suddenly "drop out" of images taken with filters bluer than z'. They suggest that these sources have redshifts which are approximately z ~ 7.
The "dropout" technique is a powerful and relatively quick way to find objects which might be at high redshift.
Copyright © Michael Richmond.
This work is licensed under a Creative Commons License.