Units to read 67,68, 69, 70, 54

A 90-100 AB 85-89 B 75 - 84 BC 70 - 74 C 60 - 69 D 50 - 59

The Lifespan of a Massive Star

Layers of Fusion Reactions As a massive star burns its hydrogen, helium is left behind, like ashes in a fireplace Eventually the temperature climbs enough so that the helium begins to burn, fusing into Carbon. Hydrogen continues to burn in a shell around the helium core Carbon is left behind until it too starts to fuse into heavier elements. A nested shell-like structure forms. Once iron forms in the core, the end is near…

Core Collapse of Massive Stars Iron cannot be fused into any heavier element, so it collects at the center of the star Gravity pulls the core of the star to a size smaller than the Earth’s diameter! The core compresses so much that protons and electrons merge into neutrons, taking energy away from the core The core collapses, and the layers above fall rapidly toward the center, where they collide with the core material and “bounce” The “bounced material collides with the remaining infalling gas, raising temperatures high enough to set off a massive fusion reaction. The star then explodes. This is a supernova!

Before and After – a Supernova

Light Curve for a Supernova The luminosity spikes at the moment of the explosion, and gradually fades, leaving behind a…

The Crab Nebula

Pulsating radio sources were discovered, and were named “pulsars” A Surprise Discovery Pulsating radio sources were discovered, and were named “pulsars” All pulsars are extremely periodic, like the ticking of a clock. But in some cases, much, much faster…

An Explanation An idea was proposed that solved the mystery A neutron star spins very rapidly about its axis, thanks to the conservation of angular momentum If the neutron star has a magnetic field, this field can form jets of electromagnetic radiation escaping from the star If these jets are pointed at Earth, we can detect them using radio telescopes. As the neutron star spins, the jets can sweep past earth, creating a signal that looks like a pulse. Neutron stars can spin very rapidly, so these pulses can be quite close together in time!

Slowing Down? Over time, the spin rate of a pulsar can decrease at a small but measurable rate Sometimes the pulsar’s diameter shrinks slightly, causing a momentary increase in the pulsar’s rotation These “glitches” are short lived, and the spin rate begins to decrease again.

Interior Structure of a Neutron Star

Black Holes It takes for a test particle infinite time to fall onto a black hole. How can black holes grow in mass?

Viewing a black hole You may be asking, “If light cannot escape a black hole, how can we see one?” If a black hole is in orbit around a companion star, the black hole can pull material away from it. This material forms an accretion disk outside of the event horizon and heats to high temperatures As the gas spirals into the black hole, it emits X-rays, which we can detect!

Light curves from a black hole binary system

The slowing of clocks has been demonstrated as well! General Relativity Einstein predicted that not only space would be warped, but time would be affected as well The presence of mass slows down the passage of time, so clocks near a black hole will run noticeably slower than clocks more distant The warping of space has been demonstrated many times, including by observations of the orbit of Mercury The slowing of clocks has been demonstrated as well!

Gravitational Redshift Photons traveling away from a massive object will experience a gravitational redshift. Their frequency will be shifted toward the red end of the spectrum

These groups are called star clusters Stars form in large groups out of a single interstellar cloud of gas and dust These groups are called star clusters Open clusters have a low density of stars – there is lots of space between the cluster’s members They can contain up to a few thousand stars in a volume 14 to 40 light years across The Pleiades is a very familiar open cluster

Some clusters are much more densely packed than open clusters. Globular Clusters Some clusters are much more densely packed than open clusters. These globular clusters can have as many as several million stars, in a volume 80 to 320 light years across!

A snapshot of stellar evolution Because all stars in a given cluster formed at the same time out of the same cloud of material, we can learn a lot about stellar evolution by examining a cluster’s stars We can locate each star in a cluster on an HR diagram and look for the “turnoff point”, the point on the main sequence above which the stars in the cluster have run out of fuel and become red giants We can deduce the age of a cluster by finding this turnoff point.

Finding a Cluster’s Age

We can use triangulation to calculate how far away objects are!

Finding the distance to the Moon by Triangulation The Moon is a relatively close object, and measuring the necessary angles is not too difficult. Other astronomical objects of interest are much farther away, and measuring the necessary angles in degrees is impractical Degrees have been sub-divided into arc-minutes and arc-seconds 1 degree = 60 arc-minutes 1 arc-minute = 60 arc seconds

A more modern way of finding the distance to the Moon Apollo astronauts left faceted mirrors behind when they returned to Earth Scientists can bounce laser beams off these mirrors, and measure the time it takes the laser pulse to travel to the Moon and back. We know the speed of light, c, so calculating the distance is easy!

Parallax As a person’s viewing location changes, foreground objects seem to shift relative to background objects This effect is called parallax, and can be used to measure the distance to closer astronomical objects

Measuring the Distance to Astronomical Objects using parallax

Just a little Trigonometry…