(Continued from previous posting)
But before we begin our exploration of the
Solar Neighborhood, we need to understand exactly what it is that we are
looking at. Just what is the Solar Neighborhood? How is it to be defined? Where does it end?
How (if at all) does it distinguish itself from the rest of the universe?
Well, that last question is perhaps the
easiest to answer. The Solar Neighborhood distinguishes itself by having us at its center; a purely arbitrary
determination which in turn leads to the answers to the remaining questions.
For like the venerable Ptolemaic System, our conception of the Solar
Neighborhood must of necessity be as geocentric as it comes. For other than
that one ultimately cosmologically irrelevant detail, it turns out that our
stellar neighborhood has nothing whatsoever to set it apart from any other
random locality within the Milky Way Galaxy.
(The star at the center of this image is an
almost identical twin of the Sun. It lies 45.3 light years away from the Solar
System at the extreme northern edge of the constellation Scorpius. Too faint to
be seen by naked eye through the light polluted skies of suburban Maryland, it
is an easy find with binoculars in early to mid summer. This is what our own
star would look like to any extraterrestrial astronomer who might be looking in
our direction.)
And that (the Milky Way) is probably the
best place to begin. Here at least, we have an easily definable object, within
which we can carve out the specifics of our own immediate vicinity.
It has always fascinated me that we know
so little for certain about the shape of our own galaxy, while we can see
countless others in exquisite detail. It was only in the 1990s that astronomers
even considered the idea that the Milky Way was a barred spiral, an idea that
was not verified observationally until as late as 2005. Not that long ago,
a fellow member of my local astronomy club reacted with astonishment to this
information, declaring that his whole understanding of reality had just been
rocked to its foundations! The problem, of course, is trying to observe such a
large object from within. In fact I
seriously doubt that we would ever have determined the shape of the Milky Way
to the degree that we have, were this the only galaxy in the universe, or were
the others somehow hidden from view. That it was a flattened disk could be
inferred by the bright band of stars so visible in midsummer, from which our
galaxy takes its name. The existence of a central core could be determined by
star counts, and by mapping the Sagittarius star clouds. But the existence of
spiral arms would likely have been hidden from us prior to the development of
radio astronomy.
But thanks to observations at multiple
wavelengths and comparisons with other galaxies more readily classifiable, we
can at last be reasonably certain that the Milky Way is a disk of 100,000 light
years (ly) diameter, composed of a dense central core enveloped in a bar of
uncertain length (estimates range from 3000 to 16,000 ly, and some astronomers
even postulate the existence of two separate bars, one within the other), which
in turn is surrounded by four spiral arms. The Perseus and Crux-Scutum arms
extend outward from the ends of the bar, while the Norma and Sagittarius are
anchored at points between the first two arms. Our own solar system resides
within a disconnected spur, nestled between the Sagittarius and Perseus arms
and variously labeled the Orion, or the Orion-Cygnus spur. The spur is roughly 10,000 ly in length and somewhat less than
4,000 ly across, the Sun being located at its approximate midpoint.
We are confident that the sun is located
near to the inner edge (i.e., the edge closest to the galactic center) of the
Orion Spur. This is obvious even to the naked eye, once one knows what to look
for. For in the summer, when the center of the Milky Way is displayed to our
view in the Sagittarius star clouds, there are relatively few bright stars in
our own vicinity in that direction. Whereas in winter, the glorious complex of
brilliant constellations that dominate the sky (Orion, Gemini, Auriga, Taurus)
visibly attests to the greater mass of the spur lying in that direction (i.e.,
away from the galactic center).
Finally, let’s take a look at the
particular segment of the Orion Spur that we happen to occupy. The dominant feature
of this region is one that can’t be seen – the Local Bubble, an area relatively
devoid of interstellar matter (0.05 atoms per cubic centimeter, only one tenth
the galactic average of 0.5 atoms/cc). Of wildly irregular shape, it averages
300 ly across, and is the result of an unknown supernova or supernovae that
occurred perhaps 20 million years ago, most likely within the Pleiades star
cluster. Our sun entered the Local Bubble in its orbit about the Milky Way
approximately 5-10 million years ago, and has traversed perhaps one half of the
distance across (these figures are of course very uncertain).
Complicating this picture is that within
the Local Bubble resides the Local Interstellar Cloud (a.k.a., the “Local
Fluff”). This is a 30 ly diameter region of relatively dense (in comparison to
the rest of the Local Bubble) interstellar medium (0.1 atoms per cubic
centimeter). This cloud appears to be material escaping from a nearby
star-forming region, the Scorpius-Centaurus Stellar Association. The 6,000
degree Celsius temperature of this cloud, coincidentally, is quite close to the
surface temperature of our own sun. (Although only the most sensitive
instruments would ever be aware of this fact. Remember, there is a difference
between temperature and heat, and the almost total lack of matter within the
cloud by Earthly standards means one would still rapidly freeze to death,
despite the 6000 degree temperature!)
So now that we have located our own sun
within its parent galaxy, we can more confidently examine its immediate
surroundings. Here is where definable structure ends, and arbitrariness sets in
with a vengeance. We must simply define
what the neighborhood is, without regard to physical structures. The Orion Spur
itself is far too large for our purposes, containing as it does more than a
billion stars, and much of it hidden from view by intervening gas clouds and
interstellar dust. Even the Local Bubble, at 300 ly diameter, is still too
large an object to be practicable, with many thousands of stars within its
boundaries. The Local Fluff, despite its manageable size, is too ill-defined an
object. And besides, it’s a fast-moving region. Our sun has been within it for
only the past 50,000 years or so, and will most likely exit it before another
20,000 have past – a mere blink of an eye, by galactic standards. Most commonly used definitions rely simply on
lists, such as “the 100 nearest stars”, or some other such number. So with
equal justification (or lack of it), I propose
to define the Solar Neighborhood by fiat and personal whim as “everything
within 12 light years of the Sun”.
Note how ludicrously geocentric such a
definition is. The very concept of light
year is based on the galactically irrelevant period of the Earth’s orbit
about the Sun. The number 12 ultimately owes its importance in our thinking to
the Moon’s orbit about the Earth (12 months in a year). And the Sun is chosen
for the center point of this defined area of space solely on the grounds of its
being our sun.
Why bring this point up? Because I wish to
impress upon the reader that there is nothing special about our immediate
neighborhood. Indeed, I can easily think of any number of areas within the
Milky Way of far more interest. Imagine for instance what it would be like to
live 12 ly distance from Deneb (the brightest star in the entire galaxy – still
one of the brightest in our own sky even at 3,000 ly distance!), or 12 ly away
from the fringes of the globular cluster M13 in Hercules. What a sight that would be!
But we’re stuck where we are, on the
fringes of a broken-off fragment of an incomplete spiral arm of a nondescript
barred spiral galaxy, near the center of a bubble of relative vacuum (compared
to the interstellar norm for the Milky Way), surrounded by… what? Well, here’s
what. Within that 24 ly diameter sphere with our sun at the center, we find 33
stars making up 21 discrete stellar systems, 16 of which are theoretically
visible from Howard County, MD, five of which can be seen with the naked eye.
We at last have a manageable observational goal – to track down and identify 16
Deep Sky Objects, a list which includes the brightest object in the sky outside
our solar system (Sirius), and the dimmest of which is a challenging 14.78
magnitude (DX Cancri).
And here they are:
Star Name Constellation
SPRING:
|
Magnitude
|
RA
|
DEC
|
Distance
(ly)
|
DX Cancri
Cancer
|
14.78
|
08h 29m 49.5s
|
+26º 46’ 37”
|
11.826
|
Lalande
21185 Ursa Major
|
7.47
|
11h 03m 20.2s
|
+35º 58’ 12”
|
8.2905
|
Wolf 359 Leo
|
13.54
|
10h 56m 29.2s
|
+07º 00’ 53”
|
7.7825
|
Ross 128 Virgo
SUMMER:
|
11.13
|
11h 47m 44.4s
|
+00º 48’ 16”
|
10.919
|
WISE
1541-2250 Libra
|
21.2*
|
15h 41m 51.6s
|
-22º 50’ 25”
|
9.3
|
Barnard’s
Star Ophiuchus
|
9.53
|
17h 57m 48.5s
|
+04º 41’ 36”
|
5.963
|
Ross 154 Sagittarius
|
10.43
|
18h 49m 49.4s
|
-23º 50’ 10”
|
9.6813
|
Struve
2398 A Draco
|
8.90
|
18h 42m 46.7s
|
+59º 37’ 49”
|
11.525
|
Struve2398 B
|
9.69
|
18h 42m 46.9s
|
+59º 37’ 37”
|
|
61 Cygni A Cygnus
|
5.21**
|
21h 06m
58.0s
|
+38º 44’ 58”
|
11.403
|
61 Cygni B
FALL:
|
6.03**
|
21h 06m
55.3s
|
+38º 44’ 31”
|
|
EZ
Aquarii A Aquarius
|
13.33
|
22h 38m 33.4s
|
-15º 18’ 07”
|
11.266
|
EZ Aquarii B
|
13.27
|
|||
EZ Aquarii C
|
14.03
|
|||
Ross 248 Andromeda
|
12.29
|
23h 41m 49.4s
|
+44º 10’ 30”
|
10.322
|
Groombridge 34 A Andromeda
|
8.08
|
00h 18m 22.9s
|
+44º 01’ 23”
|
11.624
|
Groombridge 34 B
|
11.06
|
|||
Luyten 726-8 A Cetus
|
12.54
|
01h 39m 01.3s
|
-17º 57’ 01”
|
8.7280
|
Luyten 726-8 B
|
12.99
|
|||
Tau Ceti Cetus
WINTER:
|
3.49**
|
01h 44m
04.1s
|
-15º 56’ 15”
|
11.887
|
Epsilon
Eridani Eridanus
|
3.73**
|
03h 32m
55.8s
|
-09º 27’ 30”
|
10.522
|
Procyon A Canis Minor
|
0.38**
|
07h 39m
18.1s
|
+05º 13’ 30”
|
11.402
|
Procyon B
|
10.7
|
|||
Sirius A Canis Major -1.46**
Sirius B 8.44
|
06h 45m
08.9s
|
-16º 42m 58”
|
8.5828
|
|
* indicates impossible to observe
** indicates naked eye visibility
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