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 dot-to-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: First, find the Big Dipper. 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. 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 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 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 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).