Keeble Observatory
April 2009 Sky from the Keeble Observatory
We promised last month to finish our ascent of the cosmic distance ladder by describing
the most distant light that we can observe. Our story actually begins in the 19th
century with the development of spectroscopy, i.e. the analysis of light by dispersing
it into its various wavelengths. With a degree of precision which often surprises
modern students, chemists and physicists in the 1800s were able to measure spectra
to a few parts per million. When a hot opaque surface is studied this way, we see
a continuous spectrum, i.e. “all the colors of the rainbow.” The exact shape of
this emission as a function of wavelength can be idealized as a blackbody spectrum
– and it was one of the challenges to our understanding of fundamental physics to
explain this spectrum. Classical physics failed miserably! Uniting thermodynamics
and electromagnetic theory gave a predicted spectrum which matched the observations
only at long wavelengths; at short wavelengths the discrepancy was so bad it came
to be known as the ultraviolet catastrophe. It wasn’t until 1900 that Max
Planck succeeded in deriving the correct spectrum, marking the dawn of what we now
teach as modern physics. (How he did this is another story!)
Two characteristics of the blackbody spectrum are important to this discussion.
First, the total power emitted by a hot surface is proportional to the fourth power
of the temperature, a relation known as Stefan’s Law, and which we write
as R = s T4, where s (lower case Greek sigma) is the Stefan-Boltzmann
constant. Second, the wavelength at which the blackbody spectrum has its
greatest intensity is inversely proportional to the temperature, written as lmaxT
= 3 mmK and known as Wien’s Law. lmax (lower case Greek lambda)
is the wavelength of the light at the peak of the spectrum, here in millimeters,
and T is the absolute temperature of the surface, in Kelvins. Kelvin temperatures
are relative to “absolute zero” (-273.16 ºC) with degrees the same increment as
the Celsius scale. This means that measuring the peak wavelength is the same as
measuring the temperature of the source. For example, the Sun’s spectrum peaks at
around 500 nanometers, which tells us that the temperature of the solar photosphere
is 5000 K. (The effective temperature is closer to 5800 K, but that’s also another
story!) Again, these results were well-known in the second half of the nineteenth
century.
These relations formalize what you’re likely already familiar with. For example,
as an incandescent lamp filament gets hotter, it gets both brighter and “whiter.”
When the filament is cool and barely lit, you see a dull red glow. As its temperature
is raised it emits more power at all colors, but especially at shorter wavelengths.
Recall last month’s description of the red shift expected as a light source moves
away at high speed. A blackbody spectrum which is shifted toward the red end of
the visible spectrum (and beyond) will appear cooler. The farther away that
source, the smaller the flux we receive because radiation spreads out with distance
from the source. So, a distant receding source will appear both fainter and
cooler. Coupled with the Hubble relation we introduced last month (V = Hd), we correlate
large red shifts with great distance; because the speed of light is finite, great
distances mean that we are looking back at a younger Universe.
At its formation, the Universe was extremely hot and extremely dense. It has been
expanding and cooling for some 13.7 billion years. Until it was about 100,000 years
old, it was so hot that all the gas was fully ionized and light could travel only
a fraction of a centimeter before being absorbed or scattered. That is, the universe
was opaque. Once it cooled sufficiently - to about 10,000 K - neutral atoms could
form and the whole thing became transparent. The light from that transition time
travels unimpeded until it arrives greatly red shifted at our telescopes today.
The red shift from that epoch is sufficient to move the blackbody peak from the
far ultraviolet to the microwave spectrum.
In 1948 Ralph Alpher and Robert Herman predicted that one consequence of the so-called
big bang model for the early universe would be a remnant background radiation field
at a temperature of less than 5 K. But, since the technology for observing at 2
mm wavelength did not exist, and since the big bang model was still in doubt, nobody
bothered to follow up on the prediction. In 1965, Arno Penzias and Robert Wilson
accidentally discovered that very radiation, whose temperature has since been measured
to be precisely 2.725 K. Penzias and Wilson received the 1978 Nobel Prize for their
observational work. Alpher and Herman were largely overlooked and only recently
are receiving posthumous credit for their theoretical work.
Why is this microwave radiation the most distant light we can see? (Yes,
microwaves are just another form of light, as are other radio waves, infrared, ultraviolet,
x-rays, etc.) Because, as we noted, the universe was opaque before this radiation
was allowed to travel unimpeded. We simply cannot see through the fog! So, the microwave
background is looking back in time to the point where the universe suddenly became
transparent.
Lunar phases for April: First Quarter on the 2nd, at 10:34 am
EDT; Full Moon on the 9th, at 10:56 am; Last Quarter on the 17th,
at 9:36 am; New Moon on the 24th, at 11:23 pm.
Predawn planet watchers will have a good month. Early in April we find Jupiter rising
2 hours before sunrise, to the east-southeast. Mars follows about an hour later,
with Venus bringing up the rear guard some 40 minutes before sunrise. Look for a
brilliant “star” to the east. By month’s end Jupiter will be 3 hours ahead of sunrise,
about 30 degrees above the south-southeast horizon by dawn. Mars and Venus rise
close together (about 5 degrees separation) an hour before sunrise to the east.
Fans of Saturn will be able to watch the ringed planet most of the night. Early
in the month, we see Saturn emerging from twilight about 30 degrees above the eastern
horizon, some 15 degrees below Regulus in Leo. It will set 11 hours later to the
west. Mercury is too close to the Sun for observing at the beginning of the month,
but we will watch it climb higher into the western twilight by the beginning of
May, setting an hour after the Sun passes the horizon. Saturn will be higher to
the southeast by month’s end, but still gives eight hours of good viewing before
setting.
An overhead view at mid-month, some two hours after sunset finds … not much near
zenith! Leo Minor is such a faint constellation, that you’ll not notice any bright
stars, and this is what’s at zenith. The Milky Way stretches from north to south,
arcing just above the western horizon – you’ll not notice it through haze and ground
clutter. Regulus is high to the south, and Saturn is below and to the left.
Ursa Major is high to the north, the familiar shape of the Big Dipper nearly inverted,
with the “pointer stars” and the end of the bowl standing directly above the pole
star, Polaris. Following the arc of the tail of the Bear (or handle of the Dipper)
back toward the east and you’ll find bright Arcturus in the constellation Bootes,
about 35 degrees above the horizon. If you continue the arc to the southeast, that
bright star is Spica, in the constellation Virgo. Castor and Pollux are high to
the west, well above Orion, which is making its last appearance until next winter.
Once you have your bearings, use binoculars to sweep slowly from Regulus to Castor
and Pollux. On a clear, moonless night you should encounter the Beehive Cluster
about midway on that line. A similar but shorter sweep from Regulus to Saturn should
reveal the spiral galaxy M96, again about midway between the endpoints of that line.
Starting at Spica, raising your sights to the midpoint of the imaginary line between
Saturn and Arcturus, and you’ll see M87, a large elliptical galaxy. (If you’re interested
in seeing decent images of these objects, consider using Google Earth’s “Google
Sky” feature.)
Copyright 2009
George Spagna