Download Constellations and Distances to Stars

Constellations and Distances to
Stars
What is a Constellation?
• A specific group of stars.
• Ancient people of many cultures connected
groups of stars to form patterns, like huge dotto-dot puzzles.
• These patterns are known as constellations.
• Each constellation has a myth describing how
it ended up in the sky.
How many constellations are there?
• The numbers have changed through the years.
• Early Greek astronomers named 48
constellations.
• The United States has recognized 88
constellations since 1922.
• Star maps and star finders are tools used to
tell which constellations will be seen in the sky
during each season.
• The change in the appearance of the sky is
due to the Earth’s revolution around the sun.
What About Horoscopes?
• Signs of the zodiac used in horoscopes are
actually names of constellations.
• A group of 13 constellations are known as the
Zodiac and lie along the ecliptic (the plane in
which most of our solar system lies).
The 12 of these 13 that are used in
astrological horoscopes include:
Aquarius (Jan. 20 - Feb. 18)
• Pisces (Feb. 19 - March 20)
• Aries (March 21 - April 20)
• Taurus (April 21 - May 20)
• Gemini (May 21 - June 21)
• Cancer (June 22 - July 22)
• Leo (July 23 - Aug. 22)
• Virgo (Aug. 23 - Sept. 22)
• Libra (Sept. 23 - Oct. 22)
• Scorpio (Oct. 23 - Nov. 21)
• Sagittarius (Nov. 22-Dec.20)
• Capricorn (Dec. 21 - Jan. 19)
• There is actually a thirteenth constellation that also crosses
the ecliptic, Ophiuchus (between Scorpio and Sagittarius).
Are the Big Dipper and the Little
Dipper Constellations?
• They are not constellations.
• The constellation Ursa Major contains the group
of stars commonly called the Big Dipper.
• The Big Dipper is not a constellation itself, but an
asterism, which is a distinctive group of stars.
• Another famous asterism is the Little Dipper in
the constellation Ursa Minor.
• The most famous star in Ursa Minor is Polaris,
the North Star.
Ursa Major (N. Hemisphere)
Ursa Minor (N. Hemisphere)
How can I find Polaris?
• Follow these directions:
1. First, find the Big Dipper.
2. Follow the 2 stars at the end of the cup upwards
(out of the cup of the Big Dipper). The next
bright star you will run into is Polaris. Polaris is
actually the end of the handle of the Little
Dipper.
3. Because the Earth's axis is precessing, Polaris is
only temporarily the North Star. In about 14,000
years, Vega will be the North Star and another
14,000 years after that, it will be Polaris again.
What is Precession?
• Precession is the movement a top makes as it
slows its spinning and begins to stop (it
wobbles instead of spinning evenly). This is
caused by the gravitational attraction of the
Sun and the Moon on Earth and because the
Earth is not quite spherical.
• If you were to stand on the north pole and
look up at the sky, Polaris would be at the
sky’s Zenith, straight overhead, fixed, and
unmoving.
How can we find the distance from
Earth to stars?
• Parallax
• Stars within the same constellation are not
necessarily close. They could appear to be
almost touching and actually be one trillion
kilometers apart. Very few stars are
gravitationally bound to one another.
• One way to know when a star is close to our
solar system is to measure parallax.
• Parallax is the apparent shift in the position of
an object when viewed from two different
positions.
• Example  Hold your hand at arm’s length
and look at one finger first with your left eye
only and then with your right eye only. Now
try doing the same thing with your finger
closer to your face
• What did you observe?
• The nearer an object is to the observer, the
greater its parallax.
• A star’s distance away from Earth in parsecs
can be calculated using the equation:
• When scientists discovered how far apart stars
actually are, they realized that a new unit of
measure was needed to record their differences.
Measuring star distances in kilometers would be
like measuring city distances in millimeters.
• Distances in space are often measured in light
years. A light year is the distance that light
travels in one year.
• Speed of light = c = 3 x 105 km/s (300,000 km/s)
• Calculate the length of 1 light year in
kilometers using the speed of light from
above. Be sure to use dimensional analysis
to find the answer.
• Always start by writing in the known
information (speed of light) and work your
way down to the given time from the
problem.
• Multiply across the top and divide by the
bottom.
• Now let’s use the length of a light year in
kilometers from above to calculate distances
in space in kilometers:
• The nearest star (other than the Sun) is
Proxima Centauri. This star is 4.3 light
years from Earth. How far is this distance in
kilometers?
Our galaxy, the Milky Way, is about
100,000 light years in diameter. How
far is this distance in kilometers?
The distance to the galaxy M87 in the
Virgo cluster is 50 million light years.
How far is this distance in
kilometers?
The distance to the most distant
object seen in the universe is about
18 million light years. How far is this
distance in kilometers?
The sun is only about 8 light minutes
away. How far is this in km?
• If light travels 1.80 x 107 km in one minute,
how far does it travel in 8 minutes?
• 1.80 x 107 km/min x 8 minutes = 1.44 x 108
km
The Celestial Sphere
• We locate objects on Earth by using the
coordinates latitude and longitude to explain
where they are. We can use a terrestrial
globe as a model of the Earth.
• Features of the Terrestrial Globe:
• Equator: The circle halfway between the North
and South Poles that divides the globe into
halves.
• Prime Meridian: The line that runs from pole to
pole through Greenwich, England.
• Latitude Lines: Imaginary lines parallel to the
equator.
• Longitude Lines or Meridians: Imaginary lines
parallel to the prime meridian.
Longitude
Latitude
How can we locate objects in the sky like
we do on Earth when we don’t have any
coordinates like latitude and longitude?
• There are coordinates, but they are called
declination and right ascension and we can
use a celestial sphere as a model of the sky.
The celestial sphere is simply an imaginary
extension of the terrestrial globe out into the
sky.
Features of the Celestial Sphere
• Celestial Equator: The projection of the Earth’s equator
out to the sky.
• Declination (Dec): The angular distance above or below
the celestial equator. Declination is measured in
degrees and is a projection of latitude on the terrestrial
globe.
• Vernal Equinox: The longitudinal zero point similar to
the prime meridian.
• Right Ascension (RA):The distance measured eastward
along the celestial equator from the vernal equinox.
Right ascension is measured in hours, with 1 hour (h)
equal to 15 degrees. Right ascension is a projection of
longitude on the terrestrial globe.
• Just as any city on Earth can be located by its
coordinates of longitude and latitude, any sky
object can be located on the celestial sphere
by its coordinates of right ascension and
declination.
Declination
Right Ascension
What if I want to tell someone in my own
local area where something is in the sky?
• Use local reference lines.
• Another way to locate sky objects is by using
reference lines that relate to the local position
of each observer and stay fixed with the
observer while objects pass by.
A star gazer’s local reference lines
include the following
• Zenith: The point on the celestial sphere
directly over your head.
• Celestial Horizon: The circle on the celestial
sphere 90 degrees from your zenith. You can
see only those stars that are above your
horizon.
• Celestial Meridian: The great circle passing
through your zenith and the north and south
points on your horizon. Only half of the
celestial meridian is above the horizon.
•
•
•
•
Key to Diagram:
a. Zenith
b. Celestial Meridian
c. Celestial Horizon
Can we see all of the stars at all times?
• No, the Earth is round and we can’t see
through it so we see different stars during
different parts of the year.
• From Michigan, we always see these stars
These stars rise and
set, so we sometimes
see them.
We never see these
stars from Michigan.
• The celestial equator is a projection of the
Earth’s terrestrial equator. It is tilted because
Earth is has a 23.5 o tilt of its axis.
• The Celestial horizon is 90 o from your zenith.
You can only see stars above your celestial
horizon.
• North circumpolar stars are stars that never
set. Our latitude is about 40 o north latitude.
• Because of our latitude, we see stars within 40 o of the
North celestial pole all of the time. They are always
above our horizon. These stars are located between
+50 o declination and + 90 o declination. WE ALWAYS
SEE THESE STARS!
• South circumpolar stars are stars that never rise. They
are within 40 o of the south celestial pole. They never
cross our celestial horizon. These stars are located
between –50 o declination and –90 o declination. WE
NEVER SEE THESE STARS!
• Stars located along the celestial equator, rise and set.
These stars are at + 40 o declination, which is equal to
our latitude. Therefore they pass our zenith when they
cross our celestial meridian. WE SOMETIMES SEE
THESE STARS (DEPENDING ON THE TIME OF YEAR).