Keeble Observatory
January 2009 Sky from the Keeble Observatory
With last month’s narrative, we pushed the technique of trigonometric parallax to its limit. As you recall, this was basically measuring the small angular shift in apparent positions for relatively nearby stars, and using that to calculate their distances based on simple trigonometry. But the technique is hampered by our inability to measure extremely tiny angles for distant stars, and its direct utility is exhausted before we are even out of our galactic neighborhood. To extend the “distance ladder” further, we need to digress to a discussion of how we classify stars.
The scientific study of stars truly dates to the 19th century and the development of spectroscopy as an analytic tool. Each chemical element gives off a distinctive spectrum – a mixture of discreet colors which acts as a way to identify the chemical composition of a star. However, it wasn’t until the 1930s that we were able to accurately go from the spectrum to a precise determination of the chemical makeup of stars. Surprisingly, though stellar spectra are varied (the Sun, for example shows strong evidence of sodium and calcium in its spectrum, while bright Sirius shows mostly hydrogen), it turns out that all stars have virtually the same composition. About 90% of all the atoms in stars are hydrogen, about 9% are helium, and all the other elements make up the other 1%. What’s different, and what leads to the different stellar spectra, is their surface temperature. Sirius is so hot that most elements other than hydrogen are fully ionized and thus do not exhibit their characteristic spectra. The Sun is cool enough that virtually hydrogen is still in its “ground state” and the other elements mentioned are at the right temperature to exhibit the spectra we see.
If we classify stars according to temperature, we find a nice sequence of changes in their spectra as we go from hotter to cooler stars. The classification scheme associated with this temperature trend identifies stars with a series of labels. The sequence O B A F G K M are the major spectral types, and each is subdivided into ten numerical sub-classes. (Generations of astronomy students have used the mnemonic sentence “Oh Be a Fine Girl – {or Guy} - Kiss Me!” to remember this sequence. It’s attributed to the students of Henry Norris Russell at Princeton in the 1920s.) The Sun, for example, is type G2. For nearby stars, we can not only determine spectral type, but we can also measure their intrinsic brightness, knowing that the apparent brightness falls off as one over the square of their distance. (Imagine being able to read this copy of the Herald Progress by the light of a single lamp at a distance of 5 feet. If you move to a point 10 feet away, you’d need four lamps, 15 feet away and you’d need nine. And so on.) When we make these measurements for stars close enough for parallax to directly give us the distance, we make the fortunate observation that stars of a given spectral type are also of the same intrinsic brightness! That means that if we know the spectral type (a relatively easy observation, not requiring that the star be close by) we also know its distance. Thus we can extend our distance scale to any star which is bright enough that we can measure its spectrum. The technique is called spectroscopic parallax, and it allows us to measure distances out to several hundred thousand light years. That’s still only a small step out into the larger universe, but it’s nearly a hundred times further than we can do with trigonometry.
Next month we’ll push the scale even further by looking at several “standard candles” used as distance markers over even greater scales.
Lunar phases for January: First Quarter on the 4th, at 6:56 am; Full Moon on the 10th, at 10:27 pm; Last Quarter on the 17th, at 9:46 pm; New Moon on the 26th, at 2:55 am.
Saturn is still visible in the predawn hours, high to the southwest early in the month, a bit lower as the month goes on. It rises by 10 pm each evening, so it’s also a good target through much of the night. Binoculars or a small telescope will let you admire the famous icy rings. Mercury and Jupiter return to the predawn skies late in the month – look for Mercury to rise about an hour before sunrise to the southeast. Mars is nominally in the predawn skies, but is so close to the Sun (and the horizon) that it’s unlikely you’ll be able to see it.
Jupiter begins the month close to Mercury, about 13 degrees above the southwest horizon at sunset. Both will pass through the Sun’s glare and reemerge in the predawn later in January, as noted above.
Looking toward zenith (i.e. straight overhead!) about 3 hours after sunset at mid-month finds the constellation Perseus, though it’s composed of few bright stars. The brightest, Mirfak, is just north of zenith. The constellation sits at the edge of the Milky Way, which divides the sky from southeast to northwest. Andromeda, which was at zenith last month at the same time, has now slipped to the west. Below Andromeda, in a direction largely devoid of stars is the “great square” of the constellation Pegasus.
Shifting to the east from zenith, we find the constellation Auriga (Latin for Charioteer), with its two brightest stars Capella (to the left, about 75 degrees) and Elnath (right, about 70 degrees). Below Auriga lies Gemini (the twins) with its brightest pair, Castor and Pollux. They’re not really twins – Pollux is an orange giant star with a planetary companion. Though it’s designated as beta Geminorum – supposedly the second brightest star in the constellation, it’s actually brighter. Castor is a visual binary which actually consists of six stars, though part of the system is a pair of very faint M dwarf stars which would be difficult to observe without a large telescope.
The Pleiades are south of zenith, above and to the right of Aldebaran, the brightest star in Taurus (the Bull). Below Taurus we find the familiar shape of Orion, easily the most recognizable of winter constellations. Brilliant Sirius in Canis Major (the Big Dog) is below Orion.
Copyright 2009
George Spagna