Keeble Observatory
July 2010 Sky from the Keeble Observatory
Last month we described measuring distances to stars. We applied a method known as geometric parallax, which is a fancy way of saying that we draw triangles. Knowing the size of Earth’s orbit and measuring the slight shift in direction to a star over a 6 month period, we can calculate its distance in parsecs. We can use this for stars out to about 300 light years. Now that we know the distance, what else can be learned? It turns out that we can learn a lot.
The 19th century saw the development of spectroscopy. Passing light through a prism dispersed it according to wavelength, i.e. color. (Today, we’re more likely to use a “grating” to accomplish this … notice the multicolored reflections from a CD or DVD.) We can measure the intensity as a function of wavelength to characterize the spectrum, a technique which dates to the mid-1800s. It turns out that all objects with temperatures above absolute zero emit continuous radiation across all wavelengths, with the actual distribution of energy determined by the temperature of the surface. Overall brightness is governed by Stefan’s Law, which tells us that the intensity per unit area is proportional to the fourth power of absolute temperature. This is why sunspots appear dark, even though the gas in that part of the solar surface is still over 4000 K, because the rest of the solar surface is at nearly 6000 K, making it about five times brighter. The peak intensity in the spectrum is governed by a relation known as Wien’s Law, which says that the product of the temperature and the wavelength at peak intensity is a constant. Thus, hotter objects emit a greater fraction of their energy at shorter wavelengths. From a practical standpoint, this means that if we determine the peak wavelength we know the temperature. Thus, from the spectrum we determine a star’s temperature.
But, we can know even more. Combining our knowledge of distance and temperature, we can also determine the size of the star. Stefan’s Law tells us the overall brightness at the star’s surface, and the total luminosity depends as well on its surface area. We measure its apparent brightness at our telescope, which depends on both the luminosity of the star and the inverse square of the distance. So now we know distance, temperature, and radius. In addition to the continuous part of the spectrum, we also know that each chemical element has a characteristic discrete spectrum – i.e. only certain wavelengths are present. If we look at a hot surface through a hot gas, what we see is the continuous spectrum of the surface, minus the discrete spectrum that would be emitted by the gas alone, which also depends on temperature. This means that the absorption spectrum can be used to measure the temperature and composition of a star’s atmosphere. This evidence tells us that all stars are mostly hydrogen and helium, with the rest of the chemical elements a mere trace. If we’re counting atoms or their nuclei, 90% of everything is hydrogen, and about 9% is helium.
So, now we know a star’s distance, size, and what they’re made of. Not bad for capturing a little light and asking it lots of questions. Next month we’ll discuss determining stellar masses.
Lunar phases for July: Last Quarter on the 4th, at 10:35 am; New Moon on the 11th, at 3:40 pm; First Quarter on the 18th, at 6:11 am; Full Moon on the 25th at 9:37 pm. The New Moon is accompanied by a total solar eclipse, but you’ll have to be on a boat in the South Pacific to see it!
Predawn planet watchers will find Jupiter the only bright target this month. Look for it high to the south-southeast early in July. With good binoculars or a modest telescope you may be able to find Uranus about 2 degrees to the right. Jupiter will move progressively to the west relative to the horizon, finishing the month at morning twilight about 45 degrees off the south-southwest horizon. Evenings are better for planets this month. Early in July, evening twilight finds Saturn, Mars, and Venus arrayed in a line. Saturn is at 45 degrees to the southwest, Mars south-southwest at 40 degrees, and Venus is brilliant 30 degrees above the western horizon. Mercury is west-northwest, but too low to see. Venus, Mars, and Saturn move closer on the sky, finishing the month with Mars and Saturn about a degree apart, emerging from evening twilight about 30 degrees above the south southwest horizon. Venus is 6 degrees to the right and a little below, but it will be so bright it won’t be hard to notice. Mercury will climb to about 15 degrees above the western horizon at sunset by month’s end.
Our overhead view at mid-month finds the constellation Hercules at zenith about two hours after sunset. Binoculars will allow you to pick out M13, a globular cluster orbiting our Galaxy, almost directly overhead. Sweeping our attention toward the east, we encounter Vega, the brightest star in the constellation Lyra, about 20 degrees from zenith. Binoculars or a small telescope directed to this constellation will also pick out the faint smoky Ring Nebula, the remnant of a star once much like the Sun. Below Lyra we find the familiar constellation Cygnus, about 50 degrees above the horizon. In this orientation it’s not difficult to imagine this as a stick figure of a flying swan. Deneb, to our left, marks the swan’s tail, while the colorful binary Albireo marks the swan’s head toward the right.
To the south lies bright red Antares (Mars’ rival). Its color and brightness are similar to Mars, but remember that Mars is now toward the west. To the northwest we see the familiar asterism known as the Big Dipper, which is actually part of Ursa Major. The “bowl” of the dipper is low, and the “handle” arches back toward Arcturus, a50 degrees above the west-southwest horizon. (I use this description as a mnemonic to help me remember how to identify Arcturus!). Harder to pick out is Ursa Minor, the Little Dipper, since none of this constellation’s stars are particularly bright. Try to see this dipper standing on its handle, with Polaris at the end and the smallish bowl directly above.
Copyright 2010
George Spagna