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15-1 Notes: Stars
Scientists know that the color of star indicates the star’s temperature. Red stars are
the coolest, and blue stars are the hottest. When you look at white light through a
prism, you see a rainbow of colors called a spectrum. Astronomers use a
spectroscope to separate a star’s light into a spectrum. The spectrum gives
information about the composition and temperature of a star. When a chemical
element emits light, only some colors in the spectrum appear. These are called
emission lines.
The cooler atmosphere of a star absorbs colors of light. The spectrum of a star is
called an absorption spectrum. The black lines of a star’s spectrum represent
portions of the spectrum that are absorbed by the star’s atmosphere. The spectrum
can be read to determine the elements in a star’s atmosphere. Stars are made of
mostly hydrogen and helium gas, but have traces of many other elements.
Stars are classified by how hot they are, with blue stars being the hottest and red
stars being the coolest. Stars are also classified based on brightness. Early
astronomers called the brightest stars first-magnitude stars and the dimmest stars
sixth-magnitude stars. Today, positive numbers represent dim stars, and negative
numbers represent bright stars.
The brightness of a star (as seen from Earth) is called apparent magnitude. A light
that is half as far away as another light is 4 times brighter than the other light.
Astronomers use a star’s apparent magnitude and distance from Earth to calculate
the star’s absolute magnitude. Absolute magnitude is a measure of how bright a star
would be at a distance of 32.6 light-years from Earth.
Because stars are so far away, astronomers use a unit called the light-year to
measure the distance from Earth to the stars. A light-year is the distance that light
travels in one year (about 9.46 trillion kilometers).
As Earth revolves around the sun, stars close to Earth seem to move and distant stars
seem to stay in one place. The apparent shift in position of a star is called parallax.
As you know, daytime and nighttime are caused by the rotation of Earth. Earth’s tilt
and revolution around the sun cause the seasons. Because of Earth’s rotation, the
stars appear to rotate around Polaris, the North Star. Each star is also moving in
space, but because stars are so distant, their actual motion is hard to see.
Unit 7 (Ch. 15) – Earth in the Solar System
Bach
04/02/10
Section 15-1 Review
1. What is the relationship between the color and temperature of a
star?
A star’s color indicates the star’s temperature. Blue stars are hottest;
red stars are coolest.
2. How do astronomers use spectra to determine the composition of
a star?
Each element has a unique absorption spectrum. Astronomers can
find the composition off a star by looking for the absorption spectra of
individual elements in the spectrum of a star.
3. What are two elements that commonly make up stars?
Hydrogen and helium.
4. Explain how astronomers use both positive and negative
numbers to describe star brightness.
Astronomers use positive numbers to indicate dimmer stars and
negative numbers to indicate brighter stars.
5. What is the difference between apparent magnitude and absolute
magnitude?
Apparent magnitude is the brightness of a star as seen from Earth.
Absolute magnitude is the brightness that a star would have at a
distance of 32.6 light-years from Earth.
6. Explain why astronomers use light-years rather than kilometers
to measure distances from Earth to stars.
Astronomers use light-years to measure distances from Earth to the
stars because the distances to stars are so great.
7. Light from the star Sirius takes 8.6 years to reach Earth. How far
away is Sirius?
8.6 light-years.
Unit 7 (Ch. 15) – Earth in the Solar System
Bach
04/02/10
15-1 Notes: Stars
Scientists know that the _______ of star indicates the star’s temperature. _____ stars
are the coolest, and _______ stars are the hottest. When you look at white light
through a prism, you see a rainbow of colors called a ___________. Astronomers
use a ________________ to separate a star’s light into a spectrum. The spectrum
gives information about the ______________ and temperature of a star. When a
chemical element emits ________, only some colors in the spectrum appear. These
are called ____________ lines.
The __________ atmosphere of a star absorbs colors of light. The spectrum of a star
is called an _____________ spectrum. The ________ lines of a star’s spectrum
represent portions of the spectrum that are absorbed by the star’s atmosphere. The
spectrum can be read to determine the ___________ in a star’s atmosphere. Stars are
made of mostly ____________ and __________ gas, but have traces of many other
elements.
Stars are classified by how ______ they are, with blue stars being the __________
and ______ stars being the coolest. Stars are also classified based on ____________.
Early astronomers called the ____________ stars first-magnitude stars and the
____________ stars sixth-magnitude stars. Today, __________ numbers represent
dim stars, and ____________ numbers represent bright stars.
The brightness of a star (as seen from Earth) is called _____________ magnitude. A
light that is _______ as far away as another light is 4 times brighter than the other
light. Astronomers use a star’s apparent magnitude and ____________ from Earth to
calculate the star’s absolute magnitude. ______________ magnitude is a measure of
how bright a star would be at a distance of 32.6 light-years from Earth.
Because stars are so far away, astronomers use a unit called the light-year to
measure the _____________ from Earth to the stars. A ________________ is the
distance that light travels in one year (about 9.46 trillion kilometers).
As Earth revolves around the sun, stars __________ to Earth seem to move and
_____________ stars seem to stay in one place. The apparent shift in position of a
star is called _____________.
As you know, daytime and nighttime are caused by the ____________ of Earth.
Earth’s ______ and revolution around the sun cause the ___________. Because of
Earth’s rotation, the stars appear to rotate around ___________, the North Star.
Each _______ is also moving in space, but because stars are so distant, their
__________ motion is hard to see.
Unit 7 (Ch. 15) – Earth in the Solar System
Bach
04/02/10
Section 15-1 Review
1. What is the relationship between the color and temperature of a
star?
2. How do astronomers use spectra to determine the composition of
a star?
3. What are two elements that commonly make up stars?
4. Explain how astronomers use both positive and negative
numbers to describe star brightness.
5. What is the difference between apparent magnitude and absolute
magnitude?
6. Explain why astronomers use light-years rather than kilometers
to measure distances from Earth to stars.
7. Light from the star Sirius takes 8.6 years to reach Earth. How far
away is Sirius?
Unit 7 (Ch. 15) – Earth in the Solar System
Bach
04/02/10