Fusion, Explosions, and the Search for Supernova Remnants Crystal Brogan (NRAO)

Nuclear Reactions Fission reactions split atomic nuclei –Used in nuclear reactors on earth Fusion reactions fuse atomic nuclei –The energy in stars comes from fusion

Energy Production # protons + neutrons All stars produce energy by nuclear fusion Nuclear fusion can only produce energy from elements with the number of protons + neutrons (atomic weight) less than Iron=56 otherwise it takes energy. The sun isn’t hot enough to fuse elements with higher atomic weight than Hydrogen and Helium

Periodic Table of Elements

What will happen to the Sun when it runs out of fuel? Artist ’ s conception of the formation of a white dwarf and the Helix Nebula All stars are in a constant tug-of-war between gravity inward and the energy outward from fusion

The Ring Nebula The Cat’s Eye Nebula The Stingray Nebula The Hourglass Nebula Optical HST Images of Planetary Nebula

The Hertzsprung-Russell (H-R) Diagram Main Sequence: The normal part of a star ’ s life when it is burning Hydrogen in its core. Stars have different temperatures, different luminosities, and different sizes = spectral type (OBAFGKML) Luminosity Temperature and Mass Our sun

Since they are MUCH hotter can also fuse elements up to Iron They use up all their fuel very quickly – within a few million years compared to 10 billion years for our Sun. What if the star is very massive > 8 x M sun ? It takes energy to fuse any element heavier than iron once the fuel is gone gravity wins…

What causes the explosion?  Gravity In ~1/10 second, nearly all of the iron in the core is destroyed, undoing millions of years of fusion Core collapses until it becomes as dense as material can possibly be and a neutron star or black hole is formed Infalling material from outer layers bounces off dense core In tremendous release of energy, elements heavier than iron are formed and are spread into space

Simulation of Supernova Explosion

Evolution with a companion

Over the next few days, the star will become about 100 million times brighter, often outshining all the other stars in the host galaxy combined.

The Famous Supernova of 1987: SN 1987A (closest supernova in recent history, ~160,000 l.y. away)

Radio: Very Long Baseline Array Movie of Supernova 1993J in the Galaxy M81 Timeframe of movie is 9 years (~3 frames per year)

Red: Radio Blue: X-rays Green: Optical SNR E What do Supernovae Look Like When They Get Older?  They become Supernova Remnants (SNRs) SNR Cas A Exploded in ~1670 AD The Crab Nebula SNR from 1054 AD

How Many Supernova Remnants are there in our Galaxy?  Up to the end of 2004, about 230 SNRs had been identified in our Galaxy from radio and X-ray observations How do we know this?  Massive O and B spectral type star counts  Abundance of Iron [Fe]  Observed supernova rate in the Local Group of Galaxies M51 Galaxy  However, many more SNRs are expected in our Galaxy (> 1,000) than are currently known

So What’s the Deal?  Probably due to observational selection effects  Poor resolution (hard to distinguish one thing from another)  Poor sensitivity to faint objects  Effects are most severe toward inner Galactic plane M51 Galaxy showing new Supernova Andromeda

Why Should we Care? M101 Galaxy  Important tests of our understanding of the star formation history of our Galaxy  Production of heavy elements  all elements heavier than iron on the Earth and in you come from supernova  Distribution of SNRs controls distribution of elements in the Galaxy and may be a key determinant of life on other planets SNR Cas A

Very Large Array 90cm (330 MHz) survey of 42 sq. degrees  14 pointings, each observed for ~5 hours 90cm VLA Mosaic resolution 42” Brogan et al. (2006) A Low Frequency View of the Inner Galactic Plane 11cm Bonn Survey resolution 260” Reich et al. (1984) M17 High Mass star forming region W28 Supernova Remnant

Finding the “Missing” Supernova Remnants MSX 8  m Price et al. (2001) VLA 90 cm Brogan et al. (2006) Comparing different wavelength images is the key because they show different things… 35 New SNRs discovered; a ~300% increase in this region and a 15% in the total number! Radio traces both thermal and non-thermal emission Mid-infrared traces primarily warm thermal dust emission Blue: VLA 90cm Green: Bonn 11cm Red: MSX 8  m Radio traces both thermal and non-thermal emission Mid-infrared traces primarily warm thermal dust emission

Close-Up Multi-wavelength View Blue: VLA 90cm (Brogan et al. 2006) Green: VLA + SGPS 20cm (McClure-Griffiths et al. 2005) Red: MSX 8  m (Price et al. 2001)

Summary Stars shine through nuclear fusion Stars make all elements heavier than Hydrogen When they run out of fuel : Low mass stars like the sun will turn into white dwarfs while their outer layers form planetary nebula Much more massive stars produce a supernova and supernova remnants We have not yet found the expected number of Galactic supernova remnants Comparing images at different frequencies is the key to finding more These results (35 new SNRs) suggest that a similar study of a larger part of the Galactic plane would find up to ~500 SNRs

Sources of Stellar Energy All stars produce energy by nuclear fusion of hydrogen into helium The sun isn’t hot enough to fuse heavier elements The “proton-proton” cycle =fusion of 4 Hydrogen atoms into one Helium atom: 4 H atoms = 6.693x kg 1 He atom = 6.645x kg Difference= 0.048x kg, converted to energy E=mc 2 A star is in a constant tug-of-war between gravity inward and the energy outward from fusion

Massive Stars can also use the Carbon-Nitrogen-Oxygen Cycle The CNO cycle requires much higher temperatures, but it also produces much more energy per second. Only possible in high mass stars because they are MUCH hotter The most massive stars only live a few million years compared to 10 Billion for our sun!