Review of stellar characteristics and the lifetime of typical stars

The Hertzsprung-Russell Diagram We will plot stars on a chart which has temperature on the horizontal axis and the luminosity on the vertical axis. Named after two astronomers who made this type of plot in the 1920’s. Use units of solar luminosity and Kelvin

Reminder: we need to use the HR diagram as we describe stellar evolution FIGURE 11-7 A Hertzsprung-Russell Diagram On an H-R diagram, the luminosities of stars are plotted against their spectral types. Each dot on this graph represents a star whose luminosity and spectral type have been determined. Some well-known stars are identified. The data points are grouped in just a few regions of the diagram, revealing that luminosity and spectral type are correlated: Main-sequence stars fall along the red curve, giants are to the right, supergiants are on the top, and white dwarfs are below the main sequence. The absolute magnitudes and surface temperatures are listed at the right and top of the graph, respectively. These are sometimes used on H-R diagrams instead of luminosities and spectral types. (Answer to text question: An M0 star is the next coolest after a K9.)

Here are the main regions on the HR diagram. FIGURE 11-8 The Types of Stars and Their Sizes On this H-R diagram, stellar luminosities are plotted against the surface temperatures of stars. The dashed diagonal lines indicate stellar radii. For stars of the same radius, hotter stars (corresponding to moving from right to left on the H-R diagram) glow more intensely and are more luminous (corresponding to moving upward on the diagram) than cooler stars. While individual stars are not plotted, we show the regions of the diagram in which main-sequence, giant, supergiant, and white dwarf stars are found. Note that the Sun is intermediate in luminosity, surface temperature, and radius; it is very much a middle of- the-road star.

Stellar masses determine a star’s position along the main sequence, more than other properties do.

We believe that most stars form in clusters from a single large cloud that fragments. An example of an Open Cluster is the Pleiades cluster (M45, a.k.a. “the seven sisters”).

H-R diagram of the Pleiades Open Cluster There are stars all along the main sequence, from blue giants down to red dwarfs.

Cluster Evolution on the H–R Diagram

Newborn Cluster after 10 million years Notice that there are already some red giants from massive stars that have already run out of hydrogen fuel.

Young Cluster after 600 million years Notice that the cutoff is at Type A stars and that there are already some white dwarfs.

Globular Cluster – Omega Centauri

H-R diagram of Globular Cluster Omega Centauri contains no main sequence stars with mass greater than 0.8 solar mass over 10 billion years old.

Interstellar matter Gas and dust Dust grains about 10-7 m in diameter, about the size of smoke particles Dust causes reddening of the light that passes through it, but NOT redshift. This is due to absorption of the blue components of light (more-so in UV).

Light Reddening due to absorption by dust.

FIGURE 12-4 Interstellar Reddening (a) Dust in interstellar space scatters more short-wavelength (blue) light passing through it than longer-wavelength colors. Therefore, stars and other objects seen through interstellar clouds appear redder than they would otherwise. (b) Light from these two nebulae pass through different amounts of interstellar dust and therefore they appear to have different colors. Because NGC 3603 is farther away, it appears a ruddier shade of red than does NGC 3576. (Anglo- Australian Observatory)

Interstellar gas The interstellar gas is very dilute, about ONE atom per cubic centimeter. In some places it is much denser. The distribution of gas is very uneven. It is mostly Hydrogen (90%), Helium (9%), and everything else (1%).

Molecular cloud with EGGs: Evaporating Gaseous Globules these can be seen as pillars and egg-like objects

Some properties of these nebulae: note that these are AVERAGE quantities; the nebulae are actual quite uneven in their density and temperature. Note the huge mass (hundreds to thousands of solar masses) and sizes.

Radio Emission reveals the dark dust cloud.

Hydrogen 21-cm Emission (shortwave radio)

In visible light, we can see the Horsehead Nebula, a dark dust cloud In visible light, we can see the Horsehead Nebula, a dark dust cloud. A reflection nebula is seen to the lower left of the Horsehead. This is due to scattered light from a nearby bright star. The pink nebula is an emission nebula, due to UV illumination by a hot star.

Star formation – a 7 stage process 1 – an interstellar cloud 2 – shrinking cloud fragments 3 – a fragment is the size of our solar system 4 – protostar center reaches 1,000,000 K 5 – protostar at ~10 solar radius, 4000K surf. 6 – ignition of fusion in core, now a star 7 – reaches main sequence

Orion Nebula, A closer look reveals “globules” or EGGs, some of which may contain protostars.

Several disks that may be protoplanetary disks are found after blowing up the Hubble photo. a.k.a. “proplyds”

A protostar can be plotted on the H–R diagram after reaching stage 4 A protostar can be plotted on the H–R diagram after reaching stage 4. It is heated solely due to contraction and is fairly cool, but might be 1000 times as luminous as our Sun, mostly in the infrared part of the spectrum.

Plotting a newborn star on the H–R Diagram Stage 5 – T Tauri stage – has violent surface activity and may form “jets” Stage 6 – core at 10 million K and finally get fusion, so now it is a star Stage 7 – reaches the main sequence

Pre-stellar Evolutionary Tracks for stars of other masses The minimum mass needed to get nuclear fusion and produce a real star is about 0.08 solar mass, or about 80 times the mass of Jupiter. With less mass all we get are “brown dwarfs” which are “failed stars.”

Stars with Masses between 0.08 and 0.4 times the mass of the Sun have low core temperatures, live a long time, convect helium from the core, so it mixes uniformly, and will end up composed entirely of helium. FIGURE 12-20 Fully Convective Star This drawing shows how the helium created in the cores of red dwarfs rises into the outer layers of the star by convection, while the hydrogen from the outer layers descend into the core. This process continues until the entire star is helium.

A G-Type Star is similar to our Sun (1 solar mass) The evolution is shown during an imaginary trek through space. At the end of the red giant stage, the core is small, the envelope huge, and the outcome depends on the total mass of the star.

Evolution of stars with more than 0.4 solar masses FIGURE 12-21 Evolution of Stars Off the Main Sequence (a) Hydrogen fusion occurs in the cores of main-sequence stars. (b) When the core is converted into helium, fusion there ceases and then begins in a shell that surrounds the core. The star expands into the giant phase. This newly formed helium sinks into the core, which heats up. (c) Eventually, the core reaches 108 K, whereupon core helium fusion begins. This causes the core to expand, slowing the hydrogen shell fusion and thereby forcing the outer layers of the star to contract.

Solar Composition Change During stage 7 hydrogen burning causes a build-up of helium in the star’s core. We will follow the evolution of a star like the Sun, with one solar mass.

Hydrogen Shell Burning occurs around an “ash” core, which is mostly helium, and the temperature is T = 10 million K

The hydrogen shell burning causes higher pressure on the envelope, which causes the star to expand into a Red Giant. The star follows the yellow curve on the H–R diagram. Stage 8 is the “subgiant branch” and the radius is about 3 times the solar radius. An example is the star Arcturus, M = 1.5 Msolar and R = 23 Rsolar, the luminosity is about 100 times solar.

FIGURE 12-23 The Sun Today and as a Giant (a) In about 5 billion years, when the Sun expands to become a giant, its diameter will increase a hundredfold from what it is now, while its core becomes more compact. Today, the Sun’s energy is produced in a hydrogen fusing core whose diameter is about 200,000 km. When the Sun becomes a giant, it will draw its energy from a hydrogen fusing shell that surrounds a compact helium-rich core. The helium core will have a diameter of only 30,000 km. The Sun’s diameter will be about 100 times larger, and it will be about 2000 times more luminous as a giant than it is today. (b) This composite of visible and infrared images shows red giant stars in the open cluster M50 in the constellation of Monoceros (the Unicorn). See Margin Chart Monoceros, above. (T. Credner and S. Kohle, Astronomical Institutes of the University of Bonn)

Stage 10 follows the Helium Flash, which is like a huge nuclear explosion of helium “flashing” or burning quickly into carbon at 108 K Fusion of 3 He-4 nuclei produces a C-12 nucleus plus other products Then there is the Horizontal Branch

Helium Shell Burning on the Horizontal Branch

Reascending the Giant Branch occurs in a way similar to the original move up to a giant. Burning in the H and He shells is even faster than before, so the star expands even more on this “asymptotic branch”

Planetary Nebulae form when the core can’t reach 600 million K, the minimum needed for carbon burning.

All but the core of the star blows off to form a Planetary Nebula All but the core of the star blows off to form a Planetary Nebula . A white dwarf remains.

A Planetary Nebula with the shape of a ring, 0 A Planetary Nebula with the shape of a ring, 0.5 pc across, called the “Ring Nebula”.

White Dwarf formation on the H–R Diagram Some heavier elements are formed in the last years of the burning in the shells surrounding the carbon core. H, He, C, O, and some Ne and Mg are expelled from the star as a “planetary nebula”

High-Mass stellar evolutionary tracks are quite different from the low-mass stellar evolution tracks. Notice that the core can heat up so fast that the envelope of the star tends to lag behind. Carbon fusion can start before the red giant phase.

Heavy Element Fusion - shells like an onion

A Type II Supernova is a “core collapse” and occurs when the core is finally pure iron, which cannot be fused to other elements. The core collapses to a big ball of neutrons, which causes a shock wave to bounce back outward, which blows off the entire envelope of the red giant, to form a supernova remnant.

M1 – the Crab Nebula is from a supernova seen in year A.D. 1054 The remnant is 1800 pc away and the diameter is currently 2 pc.

Supernova Light Curves fall into two types

Sirius Binary System: Sirius B is a white dwarf

Sirius B has a high mass for a white dwarf, and probably came from a mass 4 Msolar star. White dwarfs are about the size of the Earth.