May 2010 Sky from the Keeble Observatory
Last month we introduced the “H-R diagram,” named for its inventors, Ejnar Hertzsprung
and Henry Norris Russell. This graph of luminosity versus temperature (with temperature
increasing from right to left, an luminosity increasing from bottom to top) displays
clear patterns. A diagonal band from “bright and hot” to “dim and cool” marks the
so-called main sequence. Above this we find a distinct region of “bright and cool”
– and below are stars which are “dim and hot.” Imagine the graph looking something
like this unlikely combination of punctuation marks: .\’ The slash is the main sequence.
The apostrophe is the red giant branch, and the period marks the white dwarfs.
Why do we use these names? Hot surfaces give off more light per unit area than cool
ones, in proportion to the fourth power of temperature, i.e. T4. For
an extremely hot star, say 40,000 K (compared to the Sun’s 5800 K), the brightness
of each square meter is more than 2000 time brighter than the Sun’s surface. Thus,
the only way the overall luminosity can be smaller than the Sun’s is for the surface
area to be much smaller. How small is that? White dwarfs are roughly the size of
Earth, while still having mass roughly the same as our Sun! Conversely, for a red
giant to be so bright with a relatively cool surface, the star must be immense.
For example, Betelgeuse, the bright red star in the constellation Orion, would fill
our solar system out past the orbit of Mars – a radius of almost 250 million kilometers.
Leaving aside for now an explanation of just what white dwarf and red giant stars
actually are, let’s turn our attention to the main sequence. Russell thought that
this band across his graph, which represents the overwhelming majority of all visible
stars, was somehow an evolutionary sequence. That is, he assumed that stars begin
very hot and very bright, and fade to dimmer and cooler states. That turns out to
be incorrect. While it’s true that we can learn of stellar evolution by studying
H-R diagrams, the main sequence itself is actually a stage in the life of a star.
Each star will evolve to the main sequence, and after exhausting its
fuel it will evolve away from the main
Once we learn how to determine the masses of stars (that’s next month’s column)
we find that the main sequence is also a mass sequence, increasing from bottom right
to upper left. We also find that the luminosity is proportional to the cube of the
mass, which we write as . It’s difficult to understate the importance of this discovery
… since the fuel available to power a star on the main sequence is directly proportional
to its mass, we can determine that the time a star stays on the main sequence is
inversely proportional to the square of its mass. If you see a massive star, it’s
young! That means that stars are still forming, rather than star formation having
been completed long ago.
Lunar phases for May: Last Quarter on the 6th, at 12:15 am; New
Moon on the 13th, at 9:04 pm; First Quarter on the 20th, at
7:43 pm; Full Moon on the 27th at 7:07 pm.
Predawn observers will see Jupiter rise about two hours before sunrise at the beginning
of the month – extending to three hours by the end of May. Just before dawn, look
for a bright “star” to the east-southeast, about 20 degrees above the horizon early
this month, climbing to about 40 degrees above the southeast horizon by month’s
end. Mercury will start putting in an appearance just before sunrise at mid-month,
rising about an hour before the Sun by the end.
As the Sun sets, look for Venus to emerge from twilight, extremely bright about
25 degrees above the western horizon. By month’s end it will be closer to 30 degrees,
and will make an attractive triangle with Castor and Pollux above it. Mars begins
the month due south and high in the sky at sunset, drifting lower and toward the
west-south west by the last week. Saturn remains an impressive show for small telescopes
and binoculars, high to the southeast at the beginning of the month, moving to about
55 degrees above the southern horizon later.
Our overhead view at mid-month, about two hours after sunset, sees zenith nearly
devoid of stars, as it was last month. The Milky Way rings the horizon, so at zenith
we are looking out of the relatively thin disk of our home Galaxy. Starting to the
north, we see the familiar inverted “dipper” of the constellation Ursa Major. If
you follow the curve of the dipper’s handle back toward the southeast you’ll encounter
Arcturus in the constellation Bootes. Continuing your turn toward the south-southeast,
Spica lies a little lower than Arcturus, but still the brightest star in Virgo.
Saturn at this hour is almost due south. The next familiar constellation as you
turn now to the southwest is Leo, with Regulus the bright heart of the Lion. Castor
and Pollux are to the west-northwest, as you complete your turn around the sky.