CHAPTER 25 Stars and galaxies

Constellations

Ancient Greeks, Romans and other cultures saw patterns of stars in the sky called constellations

They imagined they represented mythological characters, animals or familiar objects.

Stars in constellations have no relationship to each other in space.

Modern constellations

The sky is now divided into 88 constellations

Many were named by ancient astronomers

Circumpolar constellations Many constellations circle the North star,

Polaris – Big Dipper, Little Dipper, Cepheus, etc.

Polaris is directly above the North pole Constellations appear to move because of

the Earth’s rotation As Earth orbits the Sun, different

constellations come into view. While others disappear (Orion is only seen in winter)

Circumpolar const. are visible all year

Absolute and apparent magnitude some stars seem brighter than others,

but that doesn’t mean they are closer to us or are really brighter

A star that is very dim might appear bright in the sky if it’s close to Earth, and a star that is very bright might appear dim if it is far away

Measurement in space

Measuring a star’s parallax tells us the distance to stars from our solar system

Scientists measure the apparent shift of an object from two different positions.

The closer an object is to Earth, the greater the parallax

We use light years to measure distance in space because space is so huge

Light travels 300,000 km/s or about 9.5 trillion km in one year

The nearest star to Earth, other than our Sun, is Proxima Centauri, which is 4.3 light years away, or 40 trillion km

Properties of stars

The color of a star tells us its temperature Astronomers study star properties by

looking at their spectra. Light from a star passes through a

spectroscope which breaks the light into all the colors of the spectrum

Dark lines in the spectra tell scientists which elements are in a stars atmosphere