Stars! How do we know what we know about stars? What does the color of stars indicate? What is the main sequence of stars?

Dark Lanes in Milky Way – what are they? Nebulas that give birth to stars

Stars emit energy as electromagnetic radiation

Star Color = Star Temperature Gustav Kirchoff discovered that blackbodies should emit energy as electromagnetic radiation at a particular temperature Wien’s Law – hotter objects emit most energy at shorter wavelengths. (towards blue = hotter) Cooler objects emit mostly in infrared (or longer wavelengths!) (towards red = cooler)

Stefan-Boltzmann Law E = T4 The amount of energy given off by a star, (how bright it is), is equal to its temperature raised to the 4th power. How many times brighter is a star that is 2x hotter than another star? 16x brighter!

Wien’s Law Real Life Example A piece of metal heated by a torch first becomes "red hot" as the very longest visible wavelengths appear red, then becomes more orange-red as the temperature is increased, and at very high temperatures would be described as "white hot" as shorter and shorter wavelengths come to predominate the black body emission spectrum. Before it had even reached the red hot temperature, the thermal emission was mainly at longer infrared wavelengths which are not visible; nevertheless that radiation could be felt as it warms one's nearby skin.

Iron being heated

Answer these questions in your notebook What is another example of Wien’s law from your life? Why is it important to have a system to classify stars?

Pickering’s Harvard Computers Wien’s Law eventually lead to a systematic understanding of the stars origins, evolution, inner workings, and future  Main Sequence

Main Sequence of Stars - This image shows a Hertzsprung – Russell Diagram, (H-R Diagram). -It plots star color, (spectral type/surface temp.), against its luminosity, (how much light it emits). Luminosity can indicate how much energy the star gives off.

According to the H-R Diagram, there are 3 main types of stars: Main Sequence Stars Giants & Supergiants White Dwarfs

Main Sequence Stars Stars in the long stable period of their lifespan. Energy comes from the fusion of hydrogen into helium. Most stars in the universe are main sequence stars, (approx. 90%). For these stars, the hotter they are, the brighter they are. Dwarf stars: relatively small, main sequence stars: Yellow Dwarfs: Small, main-sequence stars. Our sun is this type of star. Red Dwarfs: Small, cool, very faint main sequence stars. Low surface temp compared to other stars. Most common type of star.

Giants & Supergiants Very old stars, nearing the end of their lifespan. Much larger in size compared to main sequence stars. Many main sequence stars become these when close to the end of their life. Red Giants: Relatively old stars, 100x larger or more than their original size. Cooler than their original temp. Blue Giants: Huge, very hot, blue star. Near the end of its lifespan. Formed from a main sequence star large enough that when becoming a giant, now fuses helium at its core. Supergiants: Largest type of stars known, (some almost as large as our whole solar system!). Very rare. When these stars die: they supernova and form black holes.

Faint, Virtually Dead Stars White Dwarf: Small, dense, hot star made mostly of Carbon. It is what remains of a stars core after a red giant sheds its outer layers. Nuclear cores are depleted. Normally around the size of Earth, (but much denser & heavier). Will eventually lose their heat and become a cold, dark black dwarf. This is the eventual fate of our sun. Brown Dwarf: A “star” whose mass is too small to create nuclear fusion, (temp. and pressure at the core is not great enough). Not a very luminous star. Neutron Star: Very small, super-dense star made mostly of compressed neutrons, (a bi-product of fusion). One of the possible results of the collapse of a giant star, (much larger than sun). Just one spoonful of matter from this star would weigh 1 billion tons!!

Classification of Stars Stars are classified by their spectra, (the elements they absorb), and their temperature. There are 7 main types of stars. Ordered from hottest to coolest: Hottest O B A F G – Our Sun K M Coolest Oh Be A Fine Guy/Gal, Kiss Me!

Brightness of Stars 2 Ways a star can be bright: Extremely large Extremely hot Size overrides temperature in determining brightness Must also account for: distance from Earth!

Magnitude Apparent Magnitude: Absolute Magnitude: Measure of brightness as perceived by an observer on Earth Absolute Magnitude: A star’s brightness when measured at 10 parsecs, (a distance of about 3.26 light years). Stars closer to Earth than 10 parsecs: appear brighter than they actually are, (apparent magnitude > absolute magnitude) Stars farther from Earth than 10 parsecs: appear dimmer than they actually are, (apparent magnitude < absolute magnitude)

Magnitude Why do we need to distinguish between these 2 types of magnitude? Because how bright a star appears from Earth is not an accurate measure of how bright a star really is compared to other stars! It does not account for distance from Earth. Absolute magnitude is a way to compare all stars brightness compared to one-another regardless of their distances apart.

Inverse Square Law As objects are moved closer or further away, their brightness changes as the square of the distance they’ve moved. Ex: 2x farther away = 4x dimmer Ex: 3x closer = 9x brighter

Double Stars Many stars in the universe are part of what is called a binary star system, where 2 stars rotate around a common center of mass, (they essentially orbit one-another). About half of all stars in the universe are thought to be part of systems like these, including Polaris, (the North Star). Other stars, (double stars), only appear close together due to our vantage point from Earth. Eclipsing Binary: Some stars orbit much larger ones, and from Earth’s perspective can often appear to look like a single star that varies in brightness as the one star crosses paths with the other during orbit.