March 2009 Sky from the Keeble Observatory
We have been describing for several months the various steps along the “cosmic distance
ladder,” by which we really mean answering the question of how to measure distances
to the stars and galaxies we see in the night sky. Initially, we drew triangles
– the method of parallax, noting that directions to nearby objects shift as we change
our viewing position. From opposite sides of Earth’s orbit around the Sun, we can
detect small shifts in apparent position, and derive the distance. Unfortunately,
we also saw that this technique only works out to distances of a few hundred
We then saw that stars of a given spectral class share very nearly identical intrinsic
brightness. Relying on nearby stars for calibration, we are able to calculate distance
from the inverse square law drop in apparent brightness as we are farther away from
a given star. But even normal stars are not luminous enough to obtain these measurements
beyond perhaps a few thousand light years – hardly out of our local neighborhood.
Last month’s column introduced Cepheid variables – extremely bright variable
stars, whose intrinsic brightness is correlated to the period of their variability.
These can be seen and used as “standard candles” over distances of tens of millions
of light years. And, with each of these steps outward into the larger universe,
the precision of the techniques is reduced, so careful cross-calibration is needed.
Let’s look now at the last two steps on the ladder. In the 1920s a brilliant Indian
physicist named Subramanian Chandrasekhar calculated that there was a mass limit
for white dwarf stars. These are the remnant cores of stars up to about 5 or 6 times
the mass of the Sun. The rest of the star is ejected into space as a “planetary
nebula,” leaving the white dwarf to simply cool off over many billions of years.
Chandra, as he was popularly known until his death in 1995, determined that a white
dwarf of more than 1.44 times the mass of the Sun would collapse under its own self-gravity
and detonate in a cataclysmic nuclear explosion which would out-shine an entire
galaxy for several months. This explosion is called a Type Ia supernova,
and since these all start with a white dwarf at the mass limit, they’re all pretty
much alike and therefore reach the same peak brightness, so they make superb standard
candles for distance determination. (Variations in the exact composition of the
white dwarf generate some differences in the peak brightness, but these can be calibrated
One of the primary missions of the Hubble Space Telescope was to calibrate the Type
Ia supernova distances, which was accomplished by looking for them in the Virgo
cluster of galaxies, which lies about 50 million light years away, and whose distance
can also be determined using Cepheids. Once calibrated, the supernovae can be seen
over even greater distances … they are, after all, brighter than a galaxy!
In the 1920s, Edwin Hubble noted the following observational correlation: With only
a handful of exceptions for close-by galaxies, there is a direct proportionality
between the distance to the galaxy and how fast it is moving away from us, a relation
we can write as v = Hd. Knowing the recession velocity means that we know
the distance. That is, we solve for d = v/H. That measurement is obtained
by looking at the aggregate spectrum of the distant galaxy, which will usually be
dominated by the absorption lines of ionized calcium. Comparing the wavelengths
at which they are measured to the wavelengths for the same lines in the laboratory
tells us how fast it’s moving – this is the familiar “Doppler effect” which also
applies to sound from a moving source. An approaching source is heard shifted to
a higher note, a receding source is heard lower. For light, the higher “notes” are
bluer and the lower are redder, so we refer to a blue shift for approaching sources
and a red shift for receding sources. Incidentally, another primary mission for
HST was to calibrate this “Hubble relation” and determine the value of H - the current
accepted value is about 74 kilometers per second for every million parsecs of distance.
We can now use the red shift to determine distances to objects as far away as 5
– 7 billion light years.
Next month, we’ll look at the ultimate red shift – i.e. the most distant light we
Lunar phases for March: First Quarter on the 4th, at 2:46 am (Eastern
Standard Time – we switch to Daylight Saving Time on the 8th, so the
rest of these times are EDT); Full Moon on the 10th, at 10:38 pm; Last
Quarter on the 18th, at 1:47 pm; New Moon on the 26th, at
Venus begins the month high and brilliant at sunset, but as it catches up to us
in its orbit around the Sun, it appears closer and closer to the Sun in the sky
(think of it having the inside lane at a race track. By month’s end it will pass
in front of the Sun and move into the predawn sky. Saturn rises within an hour after
sunset early in March, about two hours after sunset by the end of the month.
Predawn planet watchers will struggle to catch Mercury and Mars passing within a
degree of one another. Unfortunately, they’re very low (about 10 degrees) to the
east-southeast at sunrise at the beginning of the month, so you’ll have trouble
picking them out of the haze and horizon clutter. Jupiter is above and to the right,
about 4 degrees away. Mars and Jupiter will move higher in the predawn sky as the
month passes. Saturn is setting at sunrise.
About two hours after sunset at mid-month, we find Castor and Pollux in Gemini just
south of zenith. The Milky Way divides the sky from north-northwest to south-southeast,
bowed just a bit to the west. To the east we see the constellation Leo, with Saturn
about 15 degrees below Regulus. Slowly sweeping your binoculars from Regulus to
Saturn, about half way between you’ll find the spiral galaxy M96. Try this on a
clear, moon-less night.
Sirius, the brightest star visible in our night sky is due south. The easily recognized
constellation Orion is above and to the right of Sirius, approximately southwest.
In the familiar “belt” region, we see three evenly spaced blue-white stars, all
approximately the same brightness. A modest telescope reveals lots of gas and dust
in the vicinity of the leftmost belt star, zeta Orionis, also known as Alnitak.
Embedded in the gas and dust is the famous Horsehead Nebula, though you need a long
camera exposure to see this as it usually is presented, a dark shape against a red
background. Your eyes are not sensitive to color when adapted to the nighttime darkness.
Avid sky watchers should look for Comet Lulin this month. It passes near Saturn
on the 23rd of February (still a week away as this is written) and is
expected to brighten to naked eye visibility – though such brightness predictions
are often wrong! It might remain too faint to see without binoculars or telescope,
or it might brighten spectacularly. Certainly with binoculars or a telescope you
should find it. It passes close to Regulus on the 28th of February. If
you watch it night to night, you’ll see it drift along the plane of the Ecliptic,
passing below the Beehive cluster in Cancer on March 5th. It will be
near delta Gemini (below Castor and Pollux) on the 17th and 18th.