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Milky Way Mysteries
Q: We are in the Milky Way galaxy itself. Then how can we
photograph our galaxy so that it seems taken from the opposite side of the Milky
Way? Photo is in this
link
[and shown in Figure 1]. (Prasanna, New York, New York)
A: "It’s quite true that we can’t photograph the whole galaxy
from outside as we simply can’t travel that far yet," says astronomer
Robert Massey of the
Royal Observatory Greenwich in London. "Even the fastest probes would take
millions of years to reach a suitable vantage point."
This spectacular picture of the Milky Way, though, does look
as if taken from far
 away
— mainly because it shows no scattering of stars in the foreground, but looks
can deceive.
An edge-on view of the heat that the Milky Way radiates, as
photographed by an infrared recorder onboard a satellite near the Sun. The
galaxy north pole is on top. Courtesy of NASA and the COsmic Background Explorer
(COBE) Project
First, from Earth located at the center’s edge, we can see
only the center of the galaxy. (See map of Figure 2.) Second, the
infrared photograph shows only the heat-light from the galaxy (not its visible
light). Furthermore, the infrared-telescope
 onboard
COBE is small and, therefore, takes only low resolution images, which
contributes to the galaxy’s distant, fuzzy appearance.
A
map
of the Milky Way, and the evidence supporting the map. Our Sun is in the
northern edge of the galaxy’s thin disk and on the inner edge of a spiral arm.
Drawing courtesy of Richard Powell
Last, the view looks distant because most of the heat comes
from the numerous stars in the galaxy’s central bulge — far from us, and this
blazing distant heat overwhelms heat from the comparatively cool stars near us.
It’s like viewing fireflies close by and then losing them in the glare of a
floodlight turned on. We lose the foreground-star heat in contrast with the
central bulge’s furnace-blast.
When we look at the galaxy from, say a Hawaiian observatory
(Figure 3), we see foreground stars. But, if we could lift the curtain of dust
between us and the central bulge — as we do when we view only its radiated heat
— then the brilliance of the bulge stars would also overwhelm the brightness of
nearby stars, and Figure 3 would
 look
more like Figure 1. Heat from the central bulge shines through the dust
that obscures the bulge’s visible light.
An image of the Milky Way, summer 2003, from Mauna Kea at
about 11,500 feet (3500 m). Courtesy of Wei-Hao Wang, Institute for Astronomy at
the University of Hawaii
"From Earth, we see the inside of our galaxy," says Massey.
"In rural areas, far from the lights of the city, and on nights when the Moon
doesn’t flood the sky with light, the Milky Way is easily visible as a misty
band that sometimes stretches right across the sky. This band is really made up
of the light of hundreds of thousands of millions of stars, each of which is too
faint to be seen individually [with the naked eye].
"The COBE satellite took the picture in 1990. Infrared light
[heat] from the centre of the galaxy cuts through the giant clouds of
dust that normally obscure our view.
"The bulge in the centre is made up of stars that cluster
around a giant black hole that helped our galaxy to form. The lines stretching
out on either side are the beginning of the disk, the flatter bit of the galaxy
that we live in."
Further Reading:
Royal Observatory Greenwich:
Galaxies
NASA:
Astronomy Picture of the Day (and Figure 1)
UC Berkeley, Lawrence Berkeley National Laboratory:
COBE, COSmic
Background Explorer
Universita di Bologna:
The Milky Way by Stefano Zattini
Anzwers.org: An Atlas of the Universe,
The Milky
Way by Richard Powell
Q: How do astronomers know how many
stars are in the Milky Way? (Mary, Springfield, Illinois)
A: The main way we calculate the number of stars in the Milky
Way sounds simple.
 We
determine the mass of our galaxy, and then divide by the mass of our Sun. That
gives us the number of Sol-mass stars in the galaxy.
The barred, spiral Milky Way. We only recently found that
our galaxy has such a pronounced, long bar. The central bar is 27,000
light-years long, and the whole galaxy is about 80 to 100 thousand light-years
across. At the bulge, it’s about 3,000 light-years thick. Courtesy of
artist R. Hurt of Spitzer Science Center, JPL-Caltech and NASA
Determining the mass of the galaxy is fairly easy. Our huge
spiral-shaped galaxy rotates like a giant pinwheel; the way it rotates tells us
its mass. We know the mass of Sol. We divide the galaxy mass by Sol’s mass, and
get the number of Sol-mass stars in the galaxy (assuming that stars comprise all
the mass of the galaxy). That number is 1 to 2 trillion Suns — and it’s wrong.
Unfortunately, our assumption is badly flawed. Most of the
galaxy mass is not luminous stars; rather it is dark matter, which
consists of brown dwarfs, neutron stars, black holes, gas, dust — and goodness
know what. We don’t know the nature of most of the galaxy’s ‘missing’ dark,
invisible mass. So, we correct this bias as best we can, knowing that "locally,
perhaps as much as 90% of the bulk of the Milky Way is dark matter", says
astronomer Robert Massey
of the Royal Observatory Greenwich in London.
We arrive at about 175 billion (175 thousand million) stars,
assuming they all have the mass of the Sun.
But, we can do better than estimating the number of Sol-like
stars in the galaxy. Actually, our sun isn’t a good representative of the galaxy
stars. Most stars have less mass than Sol. We have, however, surveyed the galaxy
to determine the number of different types of stars, and from those data, we can
estimate the average mass for a typical star, which gives us the number of
average-mass stars in our galaxy.
And the answer is — about 500 billion (500 thousand million)
stars.
By the way, in the fourth century BC, the Greek philosopher
Democritus first guessed the nature of the Milky Way: "It is a cluster of small
stars very close together." In 1610, Galileo turned his newly invented telescope
to our milky path across the sky and found that Democritus was right.
Further Reading:
Royal Observatory Greenwich:
How
many stars in a galaxy?
University of Wisconsin-Madison:
Galactic survey
reveals central bar feature
ChView:
The stars
of the Milky Way by Jo Grant and Ben Lin
Washington State University:
How many
stars in the Milky Way
Wikipedia:
The Milky Way
Wikipedia:
Democritus
Sloan Digital
Sky Survey
(Answered Jan. 3, 2006)
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