Reach for the Stars

"Reach for the Stars" Solutions, Division C

65 points total

Part I: Star ID section (25 pts):

(1pt) Orion
(1pt) Canis Major
(1pt) Gemini
(1pt) Cepheus
(1pt) Taurus
(1pt) Leo
(1pt) Saturn
(1pt) Jupiter
(1pt) Betelgeuse

(1pt) Aldebaran
(1pt) Rigel
(1pt) Pollux
(1pt) Pleiades
(2pts) M42, the Great Nebula in Orion
(2pts) M31, Andromeda Galaxy
(2pts) M1, the Crab Nebula
(3pts) M16, the Eagle Nebula
(3pts) Image: SN1987A

Part II: Knowledge Tasks (35 pts):

(1pt) What component of the solar system is located between the orbits of Mars and Jupiter?

The asteroid belt.

(2pts) Why is there no atmosphere on the Moon or Mercury?

There is not enough gravity to hold the gases in place during the hottest part of the day.

(2pts) What causes the Moon's phases?

The Sun illuminates a different 50% of the Moon than the 50% that Earth sees. The part that overlaps is the part that we see illuminated from Earth at any particular phase.

(2pts) What is the most reliable method for measuring distances to nearby stars?

Parallax. By using triangulation and observing the nearby stars from different positions along Earth's orbit we can calculate the distance to the object with simple geometry.

(2pts) On the H-R diagram there is a slightly wavy line of stars that runs diagonally down from left to right. Most stars are on that line. What is the name of that line of stars?

The Main Sequence.

(2pts) On the H-R diagram Stars in the upper right corner are both bright and "cool." How can a cool star be brighter than a hot star?

The star is bigger. Stars in the upper right corner of the H-R diagram are "Giants" thus, even though each square meter of surface my radiate less light, there is a LOT more surface area.

(2pts) At the end of the Sun's life, what type of star will it become and where will it be located on the H-R diagram.

White dwarf. These are located in the bottom left corner of the H-R diagram.

(2pts) Stars that are just entering "old age" start to pulsate. These stars reside in a region on the H-R diagram called the "instability strip." What makes these stars unstable?

They are "changing fuel." I.e., They are changing from hydrogen fusion to helium fusion. Because of the different temperatures and volumes involved the star is unstable until it adjusts to the new fuel.

(2pts) Why isn't there a lunar eclipse every month at the time of full moon?

The plane of the Moon's orbit is tilted by 5 from the plane of Earth's orbit. Since the Moon is only 0.5 wide, most of the time it is either too high or too low to be in Earth's shadow.

(2pts) Two stars are the same age and mass. Star "A" is four times as bright as star "B." Which star is further away and by how much?

Star "B" is further away by a factor of 2. Because of the 1/r² drop off of light intensity, doubling the distance decreases the brightness by a factor of 4.

(2pts) How many times brighter or dimmer is a 1^st Magnitude star as compared to a 2^nd Magnitude star? Which star is brighter?

2.5 times. The 1^st Magnitude star is brighter.

(2pts) How do we use "light" to measure the temperature of a star? The Sun's temperature is 5800 K what would be the actual value of "whatever it is" that is measured by an astronomer?

By using blackbody radiation. We measure the continuous spectrum of the star and determine which color (wavelength) is brightest. From That we can determine the temperature of the star. By Wien's law: (max. wavelength in nm) = 2.9×10⁶ / (T in Kelvin) wavelength = 500 nm.

(2pts) What data make us think that many spiral galaxies contain super-massive black holes in their cores?

Strong radio and x-ray emissions, jets of matter ejected, and/or enormous orbital velocities (as measured by the Doppler red shift and blue shift) of the stars near the core as measured by Kepler's and Newton's laws.

(3pts) What causes the seasons and why is winter cold and summer hot?

Earth's axis is tilted relative to the plane of the orbit. Thus in winter the sunlight strikes Earth at an oblique angle which spreads out the light over more surface area - the light is less intense. Also, the tilt of the axis causes the days to be shorter in winter. Hence we have both weaker light and less time in that light to warm the surface. In summer the reverse is true.

(3pts) What is "sidereal time" and, how can one use the right ascension of stars to immediately determine the current sidereal time?

Sidereal time is time measured relative to the stars rather than the Sun. (A sidereal day is about 4 minutes shorter than a mean solar day.) The current local sidereal time is exactly equal to the right ascension of any star that happens to be on the meridian (line from north to south through the overhead point, or zenith) at that moment.

(The last three problems have been removed from this web site because I plan to use them again.)

This page was last updated on March 22, 2000.