April 2010 Sky from the Keeble Observatory
“In science there is only physics. All the rest is stamp collecting.” – Ernest Rutherford is alleged to have said this, though it is more than ironic that he was awarded the Nobel Prize in chemistry! However, given the state of science in the late 19th and early 20th century, there was a grain of truth. Biology was largely taxonomy … able to distinguish between species, but unable to explain genetics. Chemistry was not far removed from the days of alchemy … even the existence of atoms was subject to question, and while there were a number of chemical “laws” there was no underlying theory to explain them. Physics had erected a mathematical superstructure which seemed to successfully explain mechanical, electromagnetic, and fluid behavior. Albeit that the superstructure didn’t quite work and had to be replaced with relativity and quantum mechanics.
Astronomy was in a similar “stamp collecting” mode. What it could explain was explained using physics, but there was no deep understanding for anything celestial other than orbital mechanics … which is explained by physics. One of the tricks of stamp collecting is to group like objects to see if there are any underlying patterns. To this end, a pair of astronomers independently thought to graph two stellar characteristics to find out how they might be related. Danish astronomer Ejnar Hertzsprung and American Henry Norris Russell were hoping to find a connection between a star’s luminosity (i.e. its brightness) and its color, the latter a measure of its temperature. This graph has become one of the most useful tools of modern astronomy, and it is now known as the Hertzsprung-Russell diagram, or H-R diagram for short. A typical H-R diagram for the nearest few thousand stars is shown (note that temperature increases to the left, and that both scales are logarithmic – increments are multiplications, not additions). The temperature scale is measured from absolute zero in kelvins. The luminosity scale is relative to the Sun’s luminosity.
Their intuition was correct. There clearly are patterns here, but what do they mean? Initially, one might expect a direct correlation. Certainly, for an incandescent lamp filament this is true; i.e. the hotter the lamp, the brighter the light. This would show up as a line rising diagonally from right to left. And, for most of the stars plotted, this is true. These stars fall along the main sequence. The Sun lies approximately in the middle of the main sequence at a temperature of about 6000 K at 1 solar luminosity. The number of stars on the main sequence declines as their luminosity and temperature increase, so that there are very few super hot and super bright stars, but lots of stars fainter and cooler than the Sun.
The surprises lie off the main sequence. Below and to the left we find extremely hot stars which do not emit a lot of light. We infer them to be very small, approximately the size of Earth! Above and to the right are very cool but extremely luminous stars. These stars are huge when compared to the Sun. A red supergiant, like Betelgeuse in Orion, has a radius roughly matching the orbit of Mars!
Russell thought the main sequence represented the lifetime of a star, beginning extremely hot and cooling off over the eons. He didn’t get that right, as we’ll discuss next month.
Lunar phases for April: Last Quarter on the 6th, at 5:37 am; New Moon on the 14th, at 8:29 am; First Quarter on the 21st, at 2:20 pm; Full Moon on the 28th at 8:18 am.
Jupiter rises about an hour before sunrise at the beginning of April, but the ecliptic makes a shallow angle with the horizon, so it is only about 10 degrees above the east-southeast horizon by the time the Sun actually rises. By the end of the month it will rise two hours before sunrise, and will be 10 degrees higher at dawn.
Evening planet watchers get a better show. Venus and Mercury start the month only a few degrees apart, respectively 17 and 15 degrees above the western horizon and easily the brightest objects in that direction. They will set about an hour after sunset. At the same time, Saturn has already risen to the east, and will be visible all night. This is an ideal time to view the ringed planet, since it has just passed opposition and is still very bright. The ring plane is tilted only slightly to the line of sight, so they are not in the best orientation. Nevertheless, this is a special target with binoculars or a small telescope. Mars is still bright, high to the southeast as it emerges from evening twilight.
By month’s end, Venus is even higher to the west, but Mercury has disappeared into the Sun’s glare. Mars and Saturn have moved westward relative to the horizon, so we find Saturn high to the southeast and Mars almost due south.
If you overhead at mid-month, about two hours after sunset, you will see zenith nearly devoid of stars. The small but not easily recognized constellation, appropriately named Leo Minor, has little to recommend it to the naked eye. Better to look just below zenith and toward the south for its more easily recognized big brother, Leo. The familiar “sickle” marks the head of the Lion, with bright Regulus at the heart. Leo is nicely framed by Saturn to the southeast and Mars to the southwest. Below Saturn is the bright star Spica, in the constellation Virgo. Further to the east lies the constellation Bootes, with its brightest star Arcturus.
Turning to the north we see the Big Dipper of Ursa Major at its highest angle, with the “pointer stars” at the end of the dipper’s bowl aligned almost directly above Polaris. Contrary to popular impressions (some of my students seem to expect the North Star to be the brightest star in the sky) Polaris does not attract your eye. To the west, Gemini is high above Orion, which is settling toward the horizon as we bid this most familiar of winter’s constellations farewell for another year.
Copyright 2010George Spagna