Our Galaxy, the Milky Way Galaxy Notes

What is a galaxy?

A collection of billions of star systems bound together by gravity

How many star systems are in the Milky Way?

About 100 billion

Why does the Milky Way have about 200 billion stars if it only has 100 billion star systems?

Many star systems are binary and contain two stars

Based on the average stellar mass, what is the estimated stellar mass of the Milky Way?

About 200 billion solar masses

What is the actual estimated total mass of the Milky Way?

Around 1 trillion solar masses, due to the presence of additional unseen matter

How are most star systems arranged in the Milky Way?

In a flat disk with spiral arms

What type of galaxy is the Milky Way?

A spiral-disk galaxy

What is the diameter of the Milky Way's galactic disk?

About 30 kiloparsecs, or roughly 100,000 light-years

What is the radius of the Milky Way's galactic disk?

About 15 kiloparsecs, or approximately 50,000 light-years

Where is our Solar System located in the Milky Way's disk?

About 8 kiloparsecs (25,000 light-years) from the galactic center—roughly halfway out

How thick is the galactic disk of the Milky Way?

About 300 parsecs, or 1,000 light-years

How does the diameter of the Milky Way's disk compare to its thickness?

The diameter is about 100 times greater than the thickness

How far is the Solar System from the top or bottom edge of the galactic disk?

About 150 parsecs, or 500 light-years, from either edge

What is the galactic bulge?

A dense, rounded collection of stars near the center of the Milky Way

What would the Milky Way resemble if viewed edge-on?

A fried egg, with the flat disk as the egg white and the central bulge as the yolk

What is the galactic halo?

A sparse, roughly spherical region of scattered stars above and below the disk

How do stars in the Milky Way move?

In large orbits around the galactic center

How do star systems in the Milky Way's disk generally move?

They orbit in roughly the same direction, giving the galaxy a sense of rotation

Why isn't the Milky Way's rotation rigid like a solid body?

Because it's made of billions of independent star systems, each following its own path

What happens to the shape of the Milky Way over time?

It slowly changes as stars move, and features like spiral arms evolve

Why do stars closer to the galactic center complete orbits faster than those farther out?

Because they have shorter distances to travel

What happens to the spiral arms of a spiral-disk galaxy over millions of years?

They disperse and reform as stars move along their orbits

What are spiral density waves?

Fluctuations in stellar density that move through the galaxy and shape the spiral arms

Besides shaping spiral arms, what else do spiral density waves affect?

The density of interstellar gas in the galaxy

Why can't we define a single rotation period for a galaxy?

Because galaxies don't rotate as solid bodies—each star system has a different orbital path and speed

What is a galactic year?

The time it takes for the Sun (or the Solar System) to complete one orbit around the galactic center

How long is one galactic year?

Roughly 200-250 million years

How old is the Milky Way?

About 10 billion years

How many rotations has the Milky Way completed in its lifetime?

Approximately 40-50 rotations

How old is our Solar System?

About 5 billion years

How many orbits has our Solar System completed around the galactic center?

About 20-25 orbits

What is the study of how galaxies form, evolve, and move called?

Galactic dynaimcs

What are scientists who study galactic dynamics called?

Galactic dynamicists

How do galactic dynamists investigate galactic structure and evolution?

By analyzing the orbital motions of individual star systems

What do theoretical galactic dynamists use to model galactic evolution?

Computer simulations of billions of stars interacting through gravity

How do observational galactic dynamicists study real galaxies?

By analyzing the light from galaxies

In a spiral-disk galaxy, why are stars on one side blueshifted and on the other side redshifted?

Because most stars orbit in the same direction—those on one side move toward us, and those on the opposite side move away

What do galactic dynamists learn by measuring redshift and blueshift in a galaxy?

Stellar speeds, which help them infer the galaxy's past structure and predict its future shape

How did galactic dynamists confirm the presence of a supermassive black hole at the center of the Milky Way?

By studying the orbits of stars near the center and calculating the gravitational forces involved

What is a supermassive black hole?

A black hole containing at least millions of solar masses

How do scientists think supermassive black holes formed?

They likely began as stellar black holes and grew over billions of years by consuming surrounding gas

What have studies of other galaxies revealed about their centers?

Every major galaxy hosts a supermassive black hole at its center

How have astronomers recently imaged supermassive black holes?

By using networks of radio telescopes acting as a single interferometer

What do radio images of supermassive black holes show?

The event horizon silhouetted against surrounding gas

When did astronomers first image the event horizon of a supermassive black hole in another galaxy?

In 2019, at the center of a galaxy about 16 megaparsecs (50 million light-years) away

When was the event horizon of the Milky Way's central black hole imaged?

In 2022

How massive is the supermassive black hole at the center of the Milky Way?

About 4 million solar masses

How does the mass of the Milky Way's central black hole compare to the mass of the galaxy?

It makes up only about 0.0004% of the galaxy's total mass

How does the Sun's mass compare to the total mass of the Solar System?

The Sun contains 99.9% of the Solar System's mass and dominates it gravitationally.

Can supermassive black holes hold galaxies together?

No, their mass is too small; another force must be responsible

What conclusion can be drawn about the force that binds galaxies?

It must come from something far more mysterious than supermassive black holes

What is a rotation curve?

A graph showing the orbital speed of stars as a function of their distance from the galactic center

What does the Milky Way's rotation curve look like at large distances from the center?

It flattens rather than decreasing

What is observed in the rotation curves of all spiral-disk galaxies?

They flatten at large distances from the galactic center

What does a flat rotation curve suggest about a galaxy's mass distribution?

Most of the mass must be spread throughout a vast sphere surrounding the visible star systems

What fraction of a galaxy's total mass is made up by its visible star systems?

Only about 10%

What fraction of a galaxy's mass is attributed to unseen material, or dark matter?

Roughly 90%

How do astronomers normally determine the composition of objects like stars and nebulae?

By using spectroscopy to analyze their light spectra

Why can't traditional spectroscopy detect most of a galaxy's mass?

Because the invisible mass neither emits nor absorbs light

Why do neutrons interact with photons despite being electrically neutral?

Because they have electromagnetic properties

What does it mean if something does not interact with photons?

It cannot be made of atoms or their components (protons, neutrons, or electrons)

What term do astronomers use for the unknown mass that doesn't interact with light?

Dark matter

What fraction of the Milky Way's mass is dark matter?

About 90%

Where is the Milky Way's dark matter located?

In a large spherical region surrounding the visible galaxy, called the dark matter halo

What is the dark matter halo?

A vast sphere of dark matter surrounding a galaxy's star systems

Why do we believe all spiral-disk galaxies have dark matter halos?

Because they all exhibit flat rotation curves, indicating similar mass distributions

What fraction of the universe's mass is normal (atomic) matter?

About 10%

What is normal (atomic) matter made of?

Atoms composed of protons, neutrons, and electrons

Why might dark matter be considered the "normal" matter of the universe?

Because it makes up the majority (90%) of the universe's mass

What is luminous mass?

Normal (atomic) matter that emits light, primarily stars

What fraction of normal matter is made of hydrogen?

About 75%

What fraction of normal matter is made of helium?

About 25%

What types of objects make up only a tiny fraction of the universe's normal matter?

Planets, moons, asteroids, and comets.

What percentage of the universe's mass is still unknown?

About 90%, in the form of dark matter

How was dark matter initially discovered?

Through its gravitational effects on normal (atomic) matter

What is one theory about the composition of dark matter?

It is composed of brown dwarf stars, called MACHOs, located in the galactic halo

What are MACHOS?

Massive Astrophysical Compact Halo Objects

What are brown dwarf stars?

Cool and dim stars that emit very little visible light but a significant amount of infrared light

Why is the MACHO theory likely incorrect?

Because brown dwarf stars emit infrared light, which should make them detectable, contradicting the idea that dark matter is invisible

Why does the MACHO theory (that dark matter is made up of brown dwarf stars) not work?

We do not detect any infrared or other photons from dark matter, and the number of brown dwarf stars required would exceed constraints on normal matter formed after the Big Bang

What do most astrophysicists agree about the composition of dark matter?

That dark matter is not composed of normal (atomic) matter

What is one alternative theory about dark matter?

That dark matter is made up of exotic quantum-mechanical particles

What is Supersymmetry?

A theory that proposes every particle of matter or antimatter has a corresponding supersymmetric particle

What are some examples of supersymmetric particles?

Electrons have sleptons, quarks have squarks, photons have photinos, gluons have gluinos, gravitons have gravitinos, and there are also winos and zinos

Have any supersymmetric particles been observed?

No, they remain purely hypothetical

What might help scientists observe supersymmetric particles?

Subatomic particle accelerators might one day be able to create these particles

What do some astrophysicists believe about the composition of dark matter?

It is composed of supersymmetric particles

What are hypothetical particles in Supersymmetric Relativistic Quantum Field Theory called?

WIMPs

What are WIMPs?

Weakly interacting massive particles

Why would WIMPs need to have a significant amount of mass?

To account for the predominance of dark matter in the universe

What are the two main theories about the composition of dark matter?

MACHOs and WIMPs

Which theory do more astrophysicists favor regarding dark matter?

The WIMP theory

Why is it a misconception that outer space is a perfect vacuum?

Because such a vacuum would violate the laws of physics—outer space contains the interstellar medium (ISM)

What is the interstellar medium (ISM)?

A diffuse gas that fills the space between stars, primarily concentrated within the galactic disk

What is the composition of the interstellar medium (ISM)?

Roughly 75% hydrogen, 25% helium, and trace amounts of other elements from the Periodic Table

How do astronomers determine the composition of the interstellar medium (ISM)?

By using spectroscopy, observing absorption and emission lines in starlight passing through or emitted by the ISM

What do absorption and emission lines in the ISM tell astronomers?

They help measure the wavelengths of these lines and identify the atoms responsible for them, revealing the ISM's composition

What forces influence the gases within the interstellar medium (ISM)?

Thermal pressures, gravitational forces, magnetic pressures, and cosmic rays

How do these forces affect the ISM gases?

They push the gases in seemingly random directions, causing some regions to be denser and others more tenuous

What is a region of the ISM that is denser than average called?

A diffuse nebula