The HERTZSPRUNG RUSSEL DIAGRAM A classification system for stars. It organizes stellar information by making a correlation between a stars luminosity (absolute magnitudes) and a its temperature. Named after Ejnar Hertzsprung (Danish) and Henry Norris Russell (United States) Developed in the 1920’s

What do you think color has to do with this diagram? Hint: Which color has the most energy and how does that energy relate to temperature?

Hotter (bluer) stars lie to the left Cooler (redder) stars lie to the right Luminosity (brightness) goes up the y-axis –Notice that the vertical scale is luminosity as fractions of a stars absolute luminosity compared to the Sun. A star that is similar in brightness to the Sun has a L value of 1. A star that is 10,000 times brighter than the Sun has a L value of One that is 100 times fainter than the Sun has a L value of Likewise the horizontal axis is plotted in terms of temperature as measured in Kelvins. Temperature goes across the x-axis. Note that the higher temperatures are to the left and cooler stars are to the right.

Spectral Class The spectral classification types were more accurate then attempts to measure the temperature of a star by its color. So often the temperature scale on the horizontal axis is replaced by spectral types, OBAFGKM. Spectral types refers to the state of the atoms on the star. This had the advantage of being more linear than temperature (nicely space letters) and contained more information about the star than just its temperature (the state of its atoms). It uses the letters O, B, A, F, G, K, M “Oh Be A Fine Girl/Guy, Kiss Me” Even though it is a scale based on stellar spectra it does contain correlations between the stellar spectra and the surface temperature of the star.. O = 30,000 KG = 6,000 K B = 20,000 KK = 4,000 K A = 10,000 KM = 3,000 K F = 7,000 K

Nebulas A nebula is the first stage in a stars life cycle. When the center of a nebula hits 10 million degrees that creates fusion. Fusion is a process that combines two hydrogen atoms and they create a new atom. A nebula is a ball of gases that will make a new main sequence star. We will discuss this later when we talk about the births and deaths of stars.

Main Sequence Stars Sun O B A F G K M 30,000 10,000 5,000 3,000

Main Sequence Stars After plotting many stars, patterns begin to emerge. One of the most noticeable is the main sequence, which is a band stretching diagonally across the H-R diagram, from top left (high temps, high luminosity, large size) to bottom right (low temps, low luminosity, small size). This shows a trend that cold stars tend to be faint, and hot stars tend to be bright. Sun 30,000 10,000 5,000 3,000 O B A F G K M

As mentioned, there are trends on the main sequence. The large, hot, and bright stars are found in the upper left of the diagram. Because of their color and size they are referred to as blue giants. The Very largest of these stars are called blue supergiants Sun blue giants 30,000 10,000 5,000 3,000 O B A F G K M

At the other end, stars are small, cool, and faint. For this reason these stars are called red dwarfs. These stars are likely to remain on the Main Sequence for the longest time. Our sun is in the middle of these two groups. Sun blue giants red dwarfs O B A F G K M 30,000 10,000 5,000 3,000

The white-dwarf region is characterized by a group of stars with very high temps and small luminosities. These stars, which are the most dense, make up about 9% of all stars. The red-giant region is characterized by a group of stars with low temperatures and high luminosities. These make up about 1% of all stars and they are the largest stars by volume. Sun blue giants red dwarfs O B A F G K M White Dwarfs Red Giants 30,000 10,000 5,000 3,000 White Dwarfs and Blue Giants

So where do stars come from anyway? Stars form inside relatively dense concentrations of interstellar gas and dust known as molecular clouds. These regions are extremely cold (temperature about 10 to 20K). Normally, molecules can not form in space because photons of starlight (energy) will break them apart. But in the centers of dark clouds, molecules like CO and H 2 (the most common molecules in interstellar gas clouds) can form because of the low temperatures. The deep cold also causes the gas to clump in to high density clouds. When the density reaches a certain point, stars form. Since the regions are dense, they are opaque to visible light and are known as dark nebula. - Since they don't shine by optical light, we must use IR and radio telescopes to investigate them.

Dark Nebulas

Star Formation Continued… Star formation begins when the denser parts of the cloud core collapse under their own weight/gravity. The cores are denser than the outer cloud, so they collapse first. As the cores collapse they fragment into clumps around 0.1 parsecs in size and 10 to 50 solar masses in mass. These clumps then form into protostars and the whole process takes about 10 millions years.

OK…Let’s ask the obvious question… How do we know this is happening if it takes so long and is hidden from view in dark clouds? 1. Most of these cloud cores have IR sources 2. Evidence of energy from collapsing protostars (potential energy converted to kinetic energy). 3. Also, where researchers do find young stars, they find them surrounded by clouds of gas, the leftover dark molecular cloud. 4. And they occur in clusters, groups of stars that form from the same cloud core.

Protowhater???? Protostar Once a clump of dense matter has broken free from the other parts of the cloud core, it has its own unique gravity and identity and it is then called a protostar. As the protostar forms, loose gas falls into its center. The infilling gas releases kinetic energy in the form of heat and the temperature and pressure in the center of the protostar goes up. As its temperature approaches thousands of degrees, it becomes a IR source.

Brown Dwarfs If a protostar forms with a mass less than 0.08 solar masses, its internal temperature never reaches a value high enough for thermonuclear fusion to begin. This failed star is called a brown dwarf, halfway between a planet (like Jupiter) and a star.

Becoming a Main Sequence Star Once a protostar starts burning hydrogen in its core, it quickly (in a few million years) becomes a main sequence star. There are three divisions in a stellar interior. They are are the nuclear burning core, convective zone and radiative zone. Energy, in the form of gamma-rays, is generated solely in the nuclear burning core. Energy is transferred towards the surface in the convective zone. The radiative zone is where energy is radiated out and away from the star.

The size of the zone depends on the mass of the star.

So what happens after the Main Sequence? As the supply of hydrogen in the core begins to decreases, the fusion rate goes down, and the amount of energy generated drops. From there thermal equilibrium takes over and as the temperature begins to drop the pressure in the fusion core will also decrease. The rest is a tug of war Because of hydrostatic equilibrium, there will be a drop in pressure, which means that the core region of the star will contract slightly. This will cause the temperature to go up again, and the fusion rate, for the remaining hydrogen in the core, jumps up (even though the core hydrogen is almost gone (a last gasp).

The Hydrogen Burning Shell The sole source of energy in a dying star. Once the hydrogen burning shell is created, the star makes a small jump off the main sequence in the HR diagram. It becomes a little brighter and a little cooler. The drop in surface temperature is because the envelope of the star expands a small amount, which increases the surface area. This increased surface area also increases the luminosity of the star. Once the last of the hydrogen is used up in the core of an aging main sequence star, fusion stops in the core and the temperature drops and the core collapses. The collapsing core converts gravitational energy (potential energy) into thermal energy (kinetic energy). This energy is directed into the hydrogen burning shell, which expands to consume more fuel in the star's interior.

The hydrogen burning shell generates more energy than the core did (it has access to a much larger volume of the star's mass) and the star increases sharply in luminosity and expands in size to become a red giant. Even though the star is brighter, produces more energy, its pressure has increased such that its surface area has become very large, and the surface temperature of the star drops.

This whole process takes several million years but, in the end, the main sequence star becomes either a red supergiant or a red giant, depending on its initial mass. Notice that where and how fast a star evolves is determined by its main sequence mass. Hot, massive stars age quickly and become red supergiants. Cooler, less massive stars live for 10 billion years, then evolve into red giants.

From Red Giant to Planetary Nebula A planetary nebula forms when a star can no longer support itself by fusion reactions in its center. The gravity from the material in the outer part of the star takes its inevitable toll on the structure of the star, and forces the inner parts to condense and heat up. The high temperature central regions drive the outer half of the star away in a brisk stellar wind, lasting a few thousand years. When the process is complete, the remaining core remnant is uncovered and heats the now distant gases and causes them to glow. red supergiants