Hands-On Activity: Can You Locate the Sun on the H-R Diagram?

Teacher and Student background

We see our daytime star, the Sun, as our closest and brightest star. The Sun is one of billions of stars in the Milky Way galaxy. All other stars are much farther away.

Our Sun, at the center of our solar system, is the closest star to Earth. Because of its proximity, the Sun is also the brightest star to us. The Sun is one star among billions in the Milky Way galaxy. All the other stars you see in the night sky are beyond our solar system and appear as pinpoints of light because they are so far away.

Stars shine through nuclear fusion and vary in size, color, mass, surface temperature, and brightness.

A star is a luminous sphere of hot gasses, mainly hydrogen and helium. Stars shine because they produce light energy through nuclear fusion (collision of atomic nuclei and formation into a new nucleus, making atoms of heavier elements) in their centers, called their cores. Gravitational forces hold stars together. Gravity, created by all the star's particles, pulls the particles toward the star's center while the pressure from the energy (created by the fusion process), along with collisions of the particles in the star, pushes them outward. The balance between the outward pressure and the inward force of gravity maintains the star's shape. Stars vary in size (diameter), color, mass, surface temperature, and brightness.

Star brightness can be measured in two ways: how bright a star appears from Earth and how bright a star actually is (as though the stars were lined up at the same distance).

Apparent magnitude is how bright the star seems to an observer from Earth while absolute magnitude is how bright the star would shine if it were lined up at a standard distance of 32.6 light years, or 10 parsecs. A light year is a measure of a very large distance, specifically, how far light can travel in one year, which is approximately 9 trillion miles. Astronomers also measure luminosity or the amount of energy (light) that a star emits from its surface. Fifth grade focuses on how the apparent magnitude of a star depends on its distance from Earth. The magnitude system of stellar brightness was developed by the Greek astronomer Hipparchus in 150 B.C. He devised a brightness scale where the brightest stars were ranked 1 and the dimmest stars ranked 6. Modern astronomers have extended that apparent magnitude scale to include much brighter stars that now include zero and negative values. Some example apparent magnitudes are the Sun at -26.7, Sirius at -1.4 (brightest nighttime star), Vega at 0.00, and the faintest naked eye star at +6.5.

One factor that affects the apparent brightness of a star is the relative distance from Earth: generally, closer stars appear brighter, while more distant stars appear dimmer.

Luminosity is a measure of a star's energy output, another factor that determines brightness. Stars with more luminosity will shine brighter because they are putting out more energy. This is similar to light bulbs you use in lamps: a 100-watt bulb will glow much brighter than a 25-watt bulb. However, if you compare stars that have the same size, temperature, and luminosity, then another important factor affecting apparent brightness is the distance from Earth. Stars closer to Earth appear brighter while more distant stars appear dimmer.