Neutron Stars and Pulsars Study Notes

ASTR350 Black Holes (Spring 2026)
Prof. Richard Mushotzky

Low Mass Stars (M < 8 M☉)
  - Most spend a long life on the main sequence converting Hydrogen (H) into Helium (He).
  - After exhausting hydrogen in the core, they enter a complex post-main sequence evolution, passing through the Red Giant phase.
  - End with a White Dwarf (M < 1.4 M☉).
High Mass Stars (M > 8 M☉)
  - Core temperatures are significantly hotter, resulting in shorter lifetimes.
  - They burn through Hydrogen quickly and begin to fuse heavier elements.
  - Structure consists of shells of successively heavier elements leading to the formation of an iron core.
  - The collapse of the iron core produces a supernova, which may leave behind a neutron star or a black hole.

Main sequence stars evolve and end their 'life' (period of nuclear burning) as compact objects.
The relationship between the main sequence lifetime (tMS) and the solar lifetime (tsun) is given by $t_{MS}/t_{sun} hickapprox (M/M_{sun})^{-2.5}$ .
Most massive stars end up as black holes or neutron stars, while the least massive become white dwarfs.
Stars with mass less than approximately 0.5 M☉ have main sequence lifetimes longer than the current age of the universe (Hubble time), hence they never become white dwarfs.

Original Mass: 18 M☉
Luminosity: Peaked at 150,000 solar luminosities during various phases of its evolution leading to the supernova event (February 23, 1987).
Evolution Timeline:
- Core hydrogen exhaustion, core helium exhaustion, ignition of nitrogen, and subsequent ignition of heavier elements leading up to the supernova.
Programs using various phases (main sequence to supernova) to study the history and evolutionary phases of the star.

Neutron stars form when the collapse of massive stars is halted by neutron degeneracy pressure opposing gravitational forces.
Typical mass is around 1.5 M☉, radius approximately 8-16 km, and central density which can reach 5 to 10 times that of nuclear equilibrium density.
If the stellar remnant exceeds a certain mass limit, it collapses into a black hole.

Supernovae are crucial for synthesizing many elements found in nature.
Their explosive blasts significantly influence the structure of the interstellar medium and serve as sources of cosmic rays.
Supernovae events are highly luminous and can be observed across vast distances, providing data for cosmological parameter determinations.

Stellar-mass black holes emerge from the evolution of massive stars with initial masses greater than 20 M☉.
Massive stars exhibit higher temperatures, blue colors, greater luminosity, and shorter lifespans compared to lower mass stars.
In their cores, stars fuse hydrogen into helium until hydrogen is depleted, at which point they transition off the main sequence.
Higher mass stars can engage in additional fusion phases until iron is formed, past which fusion in the core cannot generate energy.
White dwarfs, with a mass ranging from 0.2 to 1.3 times that of the Sun, are approximately the size of Earth and supported against gravity by electron degeneracy pressure.
The Chandrasekhar limit (approximately 1.4 M☉) defines the mass maximum that can be supported by electron degeneracy pressure; surpassing this leads to supernova events, potentially forming neutron stars or black holes.

Key Historical Figures: Jocelyn Bell and Antony Hewish (1967)
- Developed a novel radio telescope for studying quasars and discovered periodic signals, labeled as pulsars.
- The first observed pulse was noted to occur every 1.337 seconds, dramatically demonstrating that it could not originate from a standard star.
The recognition of pulsars marked significant findings in understanding neutron stars.

Pulsars emit most of their luminosity in x-rays and gamma rays, being faint in optical ranges, suggesting unique electromagnetic properties.
The regularity of pulse emissions was once thought to represent artificial signals (leading to the nickname LGM-1: Little Green Men 1).

The sensation of finding pulsars represented opening a "new window" in astronomy; enabling confirmation and exploration beyond optical observation techniques.
The management of potential alien signal hypotheses illustrated the epoch of understanding in radio astronomy.

Quiz: What is the origin of the pulsating emission detected?
  - a) Alien radio signals
  - b) Terrestrial signal misinterpreted
  - c) Rotating neutron star
  - d) Pulsating black hole

Baade and Zwicky (1934): Proposed the existence of neutron stars shortly after the neutron's discovery, theorizing their formation during supernova events.
Their proposition established foundational understanding regarding supernova remnants leading to neutron stars characterized by extremely high densities and small radii.

Pulsars are rapidly rotating neutron stars emitting electromagnetic radiation generated in strong magnetic fields.
Pulsars lose energy due to electromagnetic radiation derived from their rotation, causing them to spin down over time.

Majority of isolated neutron stars are detected as radio and gamma-ray pulsars, characterized by emitting beams of radiation from magnetic poles.
These neutron stars demonstrate high-density phenomena evidenced through their enormous gravitational pull and rotational behavior.

The average density exceeds that of atomic nuclei, and internal uniform gravity significantly alters structural composition.
Conditions in neutron stars might yield exotic matter states not replicable in terrestrial laboratories.

A specific class of neutron stars, magnetars possess exceptionally strong magnetic fields (10^{15} G).
Magnetars can undergo immense outbursts resulting in observable phenomena affecting Earth’s atmosphere; the 2004 event was a significant example.

Neutron stars serve as natural laboratories for testing general relativity (GR) and quantum mechanics due to their potential to emit contrasting forms of energy and retain significant gravitational fields.