Deep Sky Observing (Fall)
The term "deep sky" refers to objects that are faint and require large telescopes and dark skies. We have medium-sized telescopes and the view from the city is anything but dark. Even so, we can still look at some of the brightest deep sky objects and imagine how beautiful they must look under dark skies.
The observing list consists of different types of nebulae. The term "nebula" comes from a Greek word meaning cloud. That is exactly how these objects appear. There are four types of nebulae that you will observe: planetary nebulae, diffuse nebulae, a supernova remnant, and galaxies. Each type is quite different and is discussed below.
Planetary Nebulae. These objects
are so named because they are tiny (between 2 and 100" across), tend to be
circular, and are of high surface brightness. All of these characteristics
are shared by planets,
except that the surface brightness of these nebulae never even approaches that
of the planets, but they are bright as deep sky objects go. In fact, one
of them in the constellation Hydra, the sea serpent, is called "Jupiter's
Ghost". Actually, they are clouds of gas that are given off in the
latest stage of a solar-like star's life. After the star has shed it's
atmosphere in an expanding shell, the star collapses to become a white
dwarf. Although the star is tiny and very faint (only 1/10,000th as
luminous as the Sun), its surface is very hot (100,000 K or more), which allows
it to emit ultraviolet light. This ultraviolet light is capable of
ionizing (stripping the electrons off of) the gas in the shell that is about a
light year across and slowly expanding into space. The ionized gas is then
visible to an observer because the ions are constantly recombining with the
extra electrons and giving off line radiation. Thus, the shell appears to
glow. After a brief time (about 50,000 years), the shell has expanded and
the white dwarf has cooled so that it fades from view. While visible, the
shell is seen usually as a circular disk or ring around the faint star.
Diffuse nebulae. These
nebulae are called "diffuse" because they are BIG and
spread-out. Many are more than a degree across and would be easily visible
to the naked eye if they were bright enough. They are huge clouds of dust and
gas associated with star-formation regions. They are up to 100 light years
across! The mighty O and B stars that are born in these nurseries are so hot
(50,000 K on the surface) and luminous (100,000 times as luminous as the Sun),
that they are capable of ionizing parts of the entire cloud! Thus, the
cloud is lit-up by these stellar lighthouses so that we can see it.
Eventually, these O and B stars completely ionize and disperse the clouds and
reveal all of the stars that were bornin the cloud. They appear as a
coarse collection of stars, an open cluster. The cluster eventually disperses
(after about 100 million years) as each
star traces its own orbit through the Milky Way galaxy.
Supernova remnants. These
are the results of O, B, or A stars that blew up! About half of the star
is scattered out into space as a gas cloud with speeds of up to 1000 miles per
second! The remainder is left
behind in the form of an ultra dense neutron star (one teaspoon of a neutron
star would outweigh 100 million elephants!), or a black hole, where gravity is
so strong that even light cannot escape. A great example of such an
explosion, called a "supernova" is found in Taurus. This star
blew up about 4000 years ago, but the light from the explosion did not arrive at
Earth until about 900 years ago, in the year 1054 A.D. The Chinese
recorded the explosion. The ancient astronomers said that the light was so
bright that it cast shadows at night and was easily visible by day. Today
one can see a gas cloud that is 6' across and expanding at about 600 miles per
second (it takes a lifetime to notice significant changes in it however). At the
center lies a tiny, blinking pulsar (a rapidly rotating neutron star). This
nebula is an easy target under dark skies but becomes a real challenge from a
city.
Galaxies. Although these
look like gas clouds through a telescope, they are actually gigantic collections
of hundreds of billions of stars, and thousands of star formation regions.
Their cumulative light is seen to trace out spiral patterns (for spiral
galaxies), thin lines (when spiral galaxies are seen "edge-on"), and
elliptical, or circular balls (from elliptical galaxies that are shaped like
footballs or basketballs and are nearly devoid of gas and dust). They
appear as faint, smallish (10') objects because they are so far away. The
typical distance to
a fairly close galaxy is 10 MILLION light years away! The closest large
galaxy is the Andromeda galaxy which is a "mere" 2 million light years
away! We can see them at all because they contain so many stars and
because they are so huge. Many are over 100,000 light years across.
Objectives.
Requirements:
Directions: BEFORE THE LAB.
Find the objects on your star atlas.
Plan your observing session. Remember, the objects you will be looking for are FAINT.
Start with objects in the west and move east as the night progresses (otherwise the objects in the west will have set the time you get to them!). Object that are west have a lower Right Ascension. Remember to include a bright star near the object you want to find and list its coordinates.
DEEP SKY OBJECTS
M57 (Lyra), Planetary nebula, OR
M 27 (Vulpecula) Planetary nebula, OR
NGC 7662 (Andromeda)
Planetary nebula, OR
NGC 2392 (Gemini) Planetary
nebula.
M8(Sagittarius) Diffuse nebula. Use low or medium
power, OR
M17 (Sagittarius) Diffuse nebula. Use low
or medium power.
M31 and
M32 Galaxies (M32 is a
companion of M31 and close to it)
Bonus for two planetary nebulae
QUESTIONS: