Stars, starlight AND

Light Information

OUTCOME QUESTION(S):S1-4-06:How do astronomers measure the great distances in the universe?S1-4-07:What can an astronomer learn about a star by looking at light?

Vocabulary & People Astronomical Unit Light-yearApparent magnitude Absolute magnitudeElectromagnetic Energy Spectroscope

Scientific Notation (x 10X)• Express very large or very small numbers• “Power of 10” equals the number of places

the decimal was moved (+) large, (-) small

32 000 000.0 is 3.2 x 107

0.0000000055 is 5.5 x 10-9

2.6 x 105 is 260 000.02.6 x 10-5 is 0.000026

A negative exponent (10-5) means divide by 10 that many times

A positive exponent (107) means multiply by 10 that many times

Distance and the Universe• Common to use Astronomical Unit (A.U.)

1 A.U. = distance between the Earth and the Sun

Earth to the Sun: 1 AU = 1.5 x 108 km (150000000)

Common Distances:Sun - Pluto: ~ 40 AU Sun - Saturn: ~ 10 AUSun - Jupiter: ~ 5 AU Sun - Mars: ~ 1.5 AU

So Pluto is 40x farther away from the Sun than Earth

• Nearest star - 4.1 x 1013 km away from Earth!• This star is Proxima Centari

Light-year (LY):•The distance a beam of light travels in one year

Light moves outward fast – about 300,000 km/sec.•9.46 x 1012 km /year = 1 light-year•P. Centari is 4.3 light-years away

That’s 41,000,000,000,000 km!

• Distances can be deceiving: Bright stars look close, but may be very far away

Star

Approx. Distance (LY)P. Centari

4.3Sirius

8.8Betelguese

700Rigel

900Most distant known galaxy15,000,000,000

Star light takes years to get to Earth – this delay means we are looking at old “images” – it’s like looking into the

past…

Luminosity (brightness)There are two amounts (magnitudes) of brightness:

Apparent magnitude – brightness as we see it.Absolute magnitude – actually brightness.

The Sun has a higher apparent magnitude, since it

is so much closer than other stars

Star light can be used to determine temperature, composition and size (mass)

Star B looks brighter – more apparent magnitude

Star B is closer to us than Star A

Star B and A have the same absolute magnitude

Temperature of ColourColour – shows how much energy a star emits.• Colder star glows red• Hotter star glows bluish white or even blue

Colour Temperature (oC) ExampleBlue 25,000 – 50, 000Bluish-white 11,000 – 25,000 Rigel (Orion’s

belt)White 7,500 – 11,000 Sirius (brightest)Yellowish-white 6,000 – 7,500 Polaris Yellow 5,000 – 6,000 SunOrange 3,500 – 5,000Red 2,000 – 3,500 P. Centauri

(closest)Despite being cooler, the Sun is still bigger than about 95% of stars

Notice bigger stars are not necessarily hotter…but usually

brighter

Star Composition

• Scientists use a spectroscope to analyse the light energy coming from stars

• Light is a type of energy called: Electromagnetic Energy

Spectroscope – tool that splits light into a

pattern of colours, like a rainbow.

The black lines in the spectrum are used to identify the elements that make up the star.

The elements that make the star will absorb unique parts of the spectrum as energy is released.

Showing as the “black” lines of missing energy

Remember your chemistry: heated compounds give off a unique colour spectrum.

Scientists have heated elements and recorded the unique light energy

patterns

The “missing” black lines in the spectra of the Sun match with the known spectra of Hydrogen but not with

Mercury – so the Sun contains Hydrogen!

CAN YOU ANSWER THESE QUESTIONS?S1-4-06:How do astronomers measure the great distances in the universe?S1-4-07:What can an astronomer learn about a star by looking at light?

Vocabulary & People Astronomical Unit Light-yearApparent magnitude Absolute magnitudeElectromagnetic energy Spectroscope