CHARACTERISTICS OF STARS

LEARNING GOALS

I will be able to explain the colour, mass, and temperature of a star. I will be able to distinguish between absolute and apparent

magnitude.

STAR BRIGHTNESS

Luminosity: amount of energy produced each second by a star.

Luminosity is measured by comparing a star to the luminosity of our sun (luminosity = 1)

The brightest star in the night sky, Sirius, has a luminosity = 22. Sirius

Luminosity is also affected by the distance each source is from the observer.

Sirius is 22 times brighter than the sun and yet, the sun appears brighter. Why? Sirius is 9 ly away and the sun is 1.6 x 10-5 ly away

When placed at equal distances from the observer, the light with higher luminosity will appear brighter.

APPARENT & ABSOLUTE MAGNITUDE

Apparent Magnitude: The brightness of stars in the night sky as they appear from Earth. Lower magnitude = brighter celestial objects. i.e Sun >> Apparent magnitude = -27; vs. Polaris >> Apparent magnitude = 1.97

Absolute Magnitude: The brightness of stars as if they were all located 33 ly from Earth. I.e: Sun >> Absolute magnitude = 4.8 vs. Polaris >> Absolute magnitude = -3.63

Therefore, if Polaris and the Sun were side by side, Polaris would appear much brighter.

Which of these five stars is the brightest from Earth? From an equal distance away? From Earth: The Sun, apparent magnitude -26.8 From equal distance: Deneb, abs. magnitude -7.5

Star Apparent Magnitude

Absolute Magnitude

Sun -26.8 4.83Sirius -1.45 1.5Vega 0.04 0.5Betelgeuse 0.41 -5.6Deneb 1.99 -7.5

STAR COLOR & TEMPERATURE

Color of a star gives scientists an indication of the surface temperature.

Hottest stars appear bluish-white whereas cooler stars appear reddish

Our sun, with a temperature of 60000 C and a yellowish-white appearance falls midway in the table.

Blue star Rigel

COLOR AND TEMPERATURE RANGES OF SOME STARS

Color Temperature Range (0C)

Example(s)

Bluish 25000 – 50000

Zeta Orionis

Bluish-white 11000 – 25000

Rigel

Whitish 7500 – 11000

Sirius

Yellowish-white

6000-7500 Polaris

Yellowish 5000-6000 SunOrangish 3500-5000 ArcturusReddish 2000-3500 Betelgeuse

Zeta Orionis in Horsehead Nebula

THE MASS OF A STAR

The mass of our sun is 2 x 1030 kg and is referred to as one solar mass.

Scientists use this measurement to compare masses of other stars.

These masses can range from 0.1 to 120 solar masses. i.e. AI (a massive star) has a solar mass of 118!

LEARNING GOALS REVISISTED

I will be able to explain the colour, mass, and temperature of a star. I will be able to distinguish between absolute and apparent

magnitude.

YOUR TURN …

Please complete p373 #7, 8, 10, 11