Topics from the Guidelines

Massive Stars and their Evolution

HII Regions

  • An HII region is a region of ionized gas around a hot star. Hot stars emit UV radiation with enough energy to ionized hydrogen (remove its electron). Photons with wavelengths shorter than about 360 nm have enough energy to ionize hydrogen.
  • Most hot, young, massive stars are surrounded by HII regions.
  • Supernovae and planetary nebulae also produce HII regions.
  • DSO Examples: SN W49B, NGC 6357
  • H II regions show emission lines from the ionized hydrogen as some of the atoms recapture the electrons, and the electrons decay to lower energy levels. Other spectral emission lines are also produced, including lines from neutral and ionized He, N, O, and S. An example of an emission line spectrum is shown below.

Red Supergiants

  • Examples: Betelgeuse (Alpha Orionis)
  • Red supergiants are massive stars which have depleted hydrogen in their cores and are fusing heavier nuclei in their interiors.
  • Once the hydrogen is depleted, the stars’ surface temperatures drop and the stars move to the right in the HR diagram, but remain very luminous.
  • Depending on the mass of the star, the evolutionary track can be very complicated as the stars move back and forth across the top of the HR diagram as heavier and heavier atomic number atoms begin to fuse in a star’s interior

Cepheids

  • As massive stars move across the top of the HR diagram, they pass through the temperature range where they become unstable to pulsation.
  • The pulsations are caused by energy being deposited and then released in layers below the surface where hydrogen is becoming ionized.
  • The stars alternate expand and contract, becoming hotter, bluer, brighter, and smaller, and then cooler, redder, fainter, and bigger.
  • The temperature range where stars become Cepheids is about 5000 – 8000 K depending on a star’s luminosity.
  • The pulsation period correlates with the luminosity of the Cepheid, with more luminous Cepheids having longer periods.
  • Periods range from a few days for low luminosity Cepheids to about 100 days for high luminosity Cepheids
  • This correlation gives us the Cepheid period-luminosity relation (the “Leavitt Law”) which allows us to use Cepheids as “standard candles” to determine the distances to galaxies.
  • Classical Cepheids (type I) are evolved from stars in the mass range 4-20 solar masses and luminosities up to 100,000 times the luminosity of the Sun. Their periods range from days to months.
  • Type II Cepheids are evolved, metal-poor stars with low initial masses in the range of around 1 solar mass. They have lost mass during their earlier evolution and probably have masses of about 0.5 solar masses. They have evolved past the red giant stage and are near the end of their star-lives. They have periods between 1-50 days, and are typically 1.6 magnitudes fainter than classical Cepheids with the same period.

Semiregular Variables

  • Semiregular variable stars are giants or supergiants of intermediate and late spectral type showing considerable periodicity in their light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000 days, while the shapes of the light curves may be rather different and variable with each cycle. The amplitudes may be from several hundredths to several magnitudes (Wikipedia)
  • The SRV undergo a more complex pulsation behavior than long period variables like Mira, with multiple periods.
  • A sample light curve is shown at right.

Luminous Blue Variables

  • Examples: P Cygni, eta Carinae, S Doradus (in the Large Magellanic Cloud)
  • LBVs are massive unstable supergiant (or hypergiant) stars that undergo periodic outbursts and large eruptions with high rates of mass loss
  • They are usually surrounded by circumstellar material
  • This is a short-lived (usually less than a million years) in the life of a very massive stars.
  • They have temperatures in the range of 10,000 K < T <25,000 K
  • Luminosities are typically in the range 250,000 – 1,000,000 times solar
  • They are sometimes mistaken for supernovae (see the LBV light curves below).

Hypergiants

  • Hypergiants are evolved, very high luminosity, high-mass stars that across the top of the HR diagram. It is not always clear whether the different classifications represent stars with different initial conditions, stars at different stages of an evolutionary track, or are just the result of particular instabilities occurring at the time of observations. (Wikipedia)
  • Initial masses are usually > 25 solar masses, but they undergo mass loss at earlier phases of stellar evolution.
  • Hypergiants show strong hydrogen emission and their spectral lines are broadened, indicating rapid mass loss.
  • The class of hypergiants includes LBVs and blue, yellow, and red hypergiants.
  • They can reach radii 1000 times bigger than the Sun, roughly as big as the orbit of Saturn.
  • Their absolute magnitudes are typically in the range of MV= -10 to -15

Wolf-Rayet Stars

  • Wolf-Rayet Stars are evolved massive stars that have undergone significant mass loss. The mass loss has stripped off their outer layers to reveal regions below that have been subjected to hydrogen or helium burning.
  • Their absolute magnitudes are typically in the range of absolute magnitude between -5 and -10, or sometimes brighter, so 10,000 to a few hundred thousand times sbrighter than the Sun.
  • The temperatures of WR stars are hot, from about 30,000 K to 100,000 K. Because they are so hot, most of their radiation is emitted at ultraviolet wavelengths (remember Wien’s Law?).
  • Their compositions are heavily altered by the hydrogen or helium burning. They can show very high abundances of carbon, nitrogen, or oxygen, known as WC, WN, or WO stars..

Compact Objects

Neutron Stars

  • Examples (Geminga, PSR B0355+54)
  • Neutron stars are formed during core collapse supernova explosions when the stars’s core is compressed to very high density. The iron nuclei break down to simple nucleons, and the electrons are forced into the protons to form neutrons.
  • Neutron stars are a highly compressed neutron “gas” of very high density, around 1017 kg m-3 (compare to the Earth at 5000 kg m-3.
  • A typical neutron star has a mass of a few times the mass of the Sun (typically between 1 and 3.6 solar masses), but a radii of only about 10 km.
  • A neutron star can’t have a mass greater than 3.6 solar masses or it would collapse into a black hole.
  • Neutron stars usually spin fast, a result of the collapse of a rotating star into a much smaller radius (like a figure skater pulling in her arms). The rotation rates slow down as the neutron star ages. Rotation rates as fast as several hundred rotations per second or more have been found.
  • Some neutron stars emit beams of radiation as they spin, which we detect as pulses of radiation as the beam sweeps past the Earth. These objects are called pulsars. The pulses happen because (like the Earth) the pulsar’s magnetic field is not aligned with the rotation axis.

Magnetars

  • Magnetars are spinning neutron stars with extremely strong magnetic fields. Their magnetic fields are of order ~1015 gauss.
  • Magnetars are the most magnetic objects known.
  • The Earth's magnetic field in comparison is just under 1 gauss.
  • The strongest magnet you can buy is about 104 gauss.
  • The strongest artificial magnet ever created is about 5 x 105 gauss.
  • The magnetic field of the magnetar is probably a fossil field created just as the neutron star was forming.
  • The magnetic field is so strong that it distorts the shapes of atoms into long spindles.
  • About 1 in 10 neutron stars is thought to form with an ultra-strong magnetic field

Stellar Mass Black Holes

  • Black holes form during some Type II supernova explosions when the stellar core collapses. If the core is too massive to form a neutron star, it collapses to become a black hole.
  • Black holes are discovered when they are part of a binary system. Using Kepler’s laws we can estimate the mass of the components of the binary. Given the mass we estimate, we should see the light of two stars, but only one is visible. The second is dark – it emits no light. The conclusion is that it can only be a black hole.
  • In addition to the missing starlight, the presence of high energy photons from matter falling into an accretion disk surrounding the black hole (and eventually falling in) also confirms its presence in the binary system.
  • Masses are in the range from about 5 to a few dozen solar masses.
  • In 2016 the merger of two stellar mass black holes was detected by the Laser Interferometer Gravity Wave Observatory (LIGO) at its two sites, one in Washington State and the other in Louisiana.
  • The merger was detected through the specific pattern of gravitational waves known as a “chirp” when the frequency and amplitude of the waves increases as the two black holes orbit closer and closer together and faster and faster.
  • The two black holes had masses of about 36 and 29 solar masses before they coalesced.
  • LIGO continues to detect additional black hole – black hole mergers. These seem to have masses somewhat large than the black holes detected from binary stars in the Milky Way.

The masses of known black holes and neutron stars are shown in this plot.

Eclipsing Binaries

  • Eclipsing binaries are binary stars whose orbital plane is perpendicular to our line of sight. In this case, the two stars in the binary pass in front of each other so that we see an eclipse.
  • Eclipsing binaries are important – because we know the inclination, we can use them to determine the precise masses of each component.

X-ray & Gamma Ray Binary Systems

  • Examples: Circinus X-1
  • A high-mass X-ray binary(like Circinus X-1) emits strongly in X rays
  • One component is a massive star, usually an O or B star
  • The x-ray emitting component is a neutron star or black hole
  • The x-rays are produced when some of the stellar wind material from the massive star is captured by the compact object and falls into an accretion disk. The energy comes from converting gravitational potential energy to kinetic & thermal energy.
  • The optical light comes mostly from the massive star

Type II Supernovae

  • Type II supernovae result from the collapse of the iron core in massive stars when they can no longer produce energy from nuclear fusion.
  • The “Type II” designation refers to the type of feature in the spectrum of the supernova. Type II SN show hydrogen lines while Type II SN show no hydrogen lines. But note that Type Ib and Type Ic are also core collapse supernovae. Type Ia supernova are thought to result from exploding white dwarfs rather than core collapse in massive stars.
  • The stellar mass ranges that form either neutron stars or black holes after a supernova is still not well understood, and depends on the details of the explosion mechanism. It is generally thought that stars with initial masses less than about 25-40 solar masses form neutron stars and stars with higher initial masses form black holes. Stars with initial masses above about 90 solar masses may leave no remnant at all. Stars with masses less than about 8-9 solar masses generally end their lives as white dwarfs.
  • There are several different types of core collapse (Type II) supernovae shown in the chart below from Wikipedia.

Cause of collapse / Progenitor star approximate initial mass / Supernova Type / Remnant
Electron capture in a degenerate O+Ne+Mg core / 8–10 / Faint II-P / Neutron star
Iron core collapse / 10–25 / Faint II-P / Neutron star
25–40 with low or solar metallicity / Normal II-P / Black hole after fallback of material onto an initial neutron star
25–40 with very high metallicity / II-L or II-b / Neutron star
40–90 with low metallicity / None / Black hole
≥40 with near-solar metallicity / Faint Ib/c, or hypernova with gamma-ray burst (GRB) / Black hole after fallback of material onto an initial neutron star
≥40 with very high metallicity / Ib/c / Neutron star
≥90 with low metallicity / None, possible GRB / Black hole
Pair instability / 140–250 with low metallicity / II-P, sometimes a hypernova, possible GRB / No remnant
Photodisintegration / ≥250 with low metallicity / None (or luminous supernova?), possible GRB / Massive black hole