A STAR S LUMINOSITY TEMPERATURE RADIUS Photosphere Thermonuclear

A STAR’S LUMINOSITY, TEMPERATURE, & RADIUS Photosphere Thermonuclear Core Interior

THE PHOTOSPHERE OF A STAR IS A HOT, HIGH DENSITY GAS • The light that we see coming from a star is produced in its photosphere. • The photosphere is a thin, hot layer of high density gas overlying the star’s interior and thermonuclear core. • In the Sun’s case its photosphere is only 300 km thick. This compares with Earth’s atmosphere which is 100 km thick.

BY OBSERVING THE COLOR OF THE PHOTOSPHERE, YOU CAN CALCULATE ITS TEMPERATURE Wien’s Law: The temperature of the photosphere is inversely related to the wavelength of its color. Temperature in K Wavelength of the Color in Å

WAVELENGTHS OF THE COLORS OF VISIBLE LIGHT Color Violet Blue Green Yellow Orange Red-Orange Red 1 Å = 10 -10 m = 0. 1 nm Wavelength 4000 Å 4500 Å 4800 Å 5000 Å 5500 Å 6000 Å 7000 Å

The star Sirius appears blue-white. What is the temperature of the photosphere of Sirius? Wavelength of Blue Light The Sun appears yellow-white. What is the temperature of the photosphere of the Sun? Wavelength of Yellow Light

LUMINOSITY OF A STAR The luminosity of a star is the total energy of the light coming from its photosphere per second. The luminosity is related to the star’s visual magnitude, the higher the luminosity the brighter the star, and vice versa.

RELATIONSHIP BETWEEN LUMINOSITY, TEMPERATURE, & RADIUS • The luminosity of a star depends upon both the temperature and the surface area of the photosphere. • In the 19 th Century physicists determined that the luminosity is proportional to the fourth power of the temperature (T 4) and the surface area (4πR 2, where R is the radius of the star).

If the luminosity, temperature, and radius are all measured relative to the Sun, the formula for luminosity simplifies to:

A star has a radius of 1, 400, 000 km and a photospheric temperature of 17, 400 K. What is the luminosity? T Sun’s photospheric temperature (K) R Sun’s radius (km) L This star’s photosphere produces 324 times the light energy per second than does the Sun.

A star has a radius of 350, 000 km and a photospheric temperature of 2, 900 K. What is the luminosity? T R L This star’s photosphere produces only 1. 6% the light energy per second than does the Sun.

THE LUMINOSITY FORMULA CAN BE REORGANIZED Luminosity Radius Temperature

A star has a luminosity of 100 L and a photospheric temperature of 17, 400 K. What is the radius? T L R Radius of the star in km

Betelgeuse is a super red giant star in the constellation Orion (“The Hunter”). According to the back of your book it has a luminosity of 14, 000 L. What is the radius of this dying star? Wien’s Law λ = 7, 000 Å Wavelength of red light.

T L R Think about it. If Betelgeuse were placed where the Sun is right now, it would engulf Earth!

Sirius is a bright blue-white star in the constellation Canis Major (“The Big Dog”). According to the back of your book it has a luminosity of 23. 5 L. What is the radius of this neighboring star located 6 ly from Earth? Wien’s Law λ = 4, 500 Å Wavelength of blue light.

T L R Sirius is a star much larger than the Sun. In fact, 43 = 64 suns could fit inside of this large star.

Sirius has a companion star named Sirius B. This star was discovered in the 1800’s. According to your book, it has a luminosity of 0. 04 L and a photospheric temperature of 24, 000 K. What is the radius of this star? T L R Compare the radius of Sirius B to Earth’s radius of 6, 400 km. This star is about the same size as Earth. It is what astronomers call a white dwarf, and will be the fate of the Sun in about 7 billion years.

RELATIVE VOLUME • The relative volume of two objects is found by dividing one volume into the other volume. • Relative volume measures how many of the bottom objects will fit inside of the top object. Top Volume

Volume of a Sphere Relative Volume for two spheres.

The radius of the Sun is 700, 000 km compared to Earth’s radius of 6500 km. What is the volume of the Sun relative to Earth? The Sun is large enough to fit 1. 3 million earth’s inside of it.

The radius of Sirius is 2, 751, 000 km compared to the Sun’s radius of 700, 000 km. What is the volume of Sirius relative to the Sun? V Sirius is large enough to fit 61 sun’s inside of it.

The radius of Betelgeuse is 162, 400, 000 km compared to the Sun’s radius of 700, 000 km. What is the volume of Betelgeuse relative to the Sun? V Betelgeuse is a huge, bloated star. It is large enough to fit 12. 5 million sun’s inside of it.

The radius of Sirius B is 8, 400 km compared to the Sun’s radius of 700, 000 km. What is the volume of the Sun relative to the Sirius B? Sirius B is about the same size as Earth. The Sun can house 580 thousand of this tiny star.