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.