Universe and Stars

Scale of the Universe and

Characteristics of Stars

Chapter 7 Lessons 3 & 4

What do you think this quote means?

So how big is our universe?

​Well.......it's big.

​Very big.

large.

​Our universe is about 93 BILLION light years in diameter

A light year is...

• The distance that light travels in one year

• **A light year is a unit of DISTANCE, not time **

• This is about 9.5 TRILLION kilometers

• that's 9,500,000,000,000 or 9.5 x 10^12 km

• Scientists use scientific notation to write these huge numbers in a shorter form

Scientific Notation Review

• Insert a decimal into the original number so that it creates a number between 1 and 10. Ex: 97,850 = 9.7850

• Round the decimal to one place. Ex: 9.7850 = 9.8

• Count the number of places the decimal moved to get the power of 10

• In 9.7850, the decimal moved 4 places

• 9.7850 = 9.8 x 10^4

Multiple Choice

Convert 6,320 to scientific notation.

• 6.3 x 10^4

• 632 x 10^3

• 6.4 x 10^2

• 6.3 x 10^3

Multiple Choice

Convert 7,800,000 to scientific notation.

• 78 x 10^5

• 7 x 10^6

• 7.8 x 10^6

• 7.8 x 10^5

Earth's Neighbors

• Our closet neighboring star is Proxima Centauri at 4.25 light years away

• The closest galaxy to us is the Andromeda galaxy at 2.5 million light years away

How do we know how far away Proxima Centauri, and other stars, are?

• Astronomers often use parallax to measure distances to nearby stars

  • Parallax is the apparent change in position of an object when you look at it from different places

How we measure parallax:

• First, astronomers look at a nearby star when Earth is on one side of the Sun

• Then they look at the same star again six months later, when Earth is on the opposite side of the Sun

• Then they measure how much the star appears to move against a background of much farther away stars

• The less the star appears to move, the farther away it is

• *This only works up to a few hundred light years away

Speaking of stars....

Are they all the same? Can we classify them differently?

Classification of Stars

• Stars can be classified by:

• Color

• Temperature

• Size

• Composition

• Brightness

Color and Temperature

• The color of a star indicates the surface temperature

• The coolest stars, around 5,800 Fahrenheit, appear red

• Our yellow Sun is around 9,900 Fahrenheit

• The hottest stars, over 36,000 Fahrenheit, appear blue

Size

• In the night sky, most stars appear to be the same size

• There are a lot of stars that are similar in size to our Sun

• Some stars are much larger than our Sun; we call these Giant or Supergiant stars

• Most however are smaller than the Sun; White Dwarf stars or Neutron stars

Chemical Composition

• Astronomers use spectrographs to determine the elements found in stars

• A spectrograph is a device that breaks light into colors and produces an image of the resulting spectrum

• How does this work?

Spectrograph

• Gases in a star's atmosphere absorbs wavelengths of light from within the star

• When this light is seen through a spectrograph, each wavelength appears as a dark line on a spectrum

• Each element absorbs light at a distinct wavelength

​Hydrogen

Helium

​Helium

Calcium

Hydrogen

Sodium

Brightness of Stars

• The brightness of a star depends upon both its size and temperature

• The brightness is described in two ways:

• Apparent Brightness: its brightness as seen from Earth

• Absolute Brightness: the brightness the star would have if it were at a standard distance from Earth

Apparent Brightness

• Easily measured with devices

• Doesn't tell exactly how much light is given off from the star

• The closer to Earth the star is, the brighter it will appear

Absolute Brightness

• First astronomers need to know the star's apparent brightness AND distance from Earth

• This allows them to calculate the absolute brightness; the amount of light actually given off

• The brightest stars are more than a billion times brighter than the dimmest stars!

H-R Diagram

• This is a graph that shows the correlation between stars' color, absolute brightness, and surface temperature

• This allows astronomers to classify and understand how stars change over time

Multiple Choice

How far across is the universe?

• 98 billion light years

• 93 billion light years

• 9.5 trillion light years

Multiple Choice

What does a stars color indicate?

• Size

• Chemical Composition

• Surface temperature

• Brightness

Multiple Select

The brightness of a star depends on what 2 things?

• Size

• Color

• Composition

• Temperature

Multiple Select

What characteristics are used to classify stars?

• Composition

• Size

• Brightness

• Temperature

• Color