Measuring Distance to Distant Galaxies

Measuring Distances to Distant Galaxies

This week's topic: Distant galaxies and methods for measuring distances to them.
Parallax is effective for measuring distances to nearby stars within our galaxy (specifically, within our spiral arm).

The problem: How do we measure distances to objects very far away, like distant galaxies or objects on the other side of our galaxy?

The approach: Find an object with a known intrinsic brightness.
Inverse Square Law: The observed brightness decreases with distance according to the inverse square law.
- If we know the object's intrinsic brightness (luminosity) and measure its observed brightness (flux), we can calculate the distance.
Stars as light sources:
- Stars produce their own light (unlike planets).
- Stars emit energy (power), which astronomers call luminosity.
- Luminosity is measured in watts.

Imagine a 100-watt light bulb.
If you know its power (100 watts) and measure its brightness (flux) at a certain distance (e.g., 1 meter), you can calculate the distance.
Astronomers use this principle to measure distances to stars, but the key is knowing the star's intrinsic power (luminosity).

Cepheid variables are a specific type of variable star used as standard candles.
They exhibit a relationship between their period of variability (the time it takes to cycle through brighter and fainter phases) and their luminosity.
- By measuring the period of a Cepheid, we can determine its intrinsic luminosity.
- Knowing the intrinsic luminosity and measuring the observed brightness allows us to calculate the distance using the inverse square law.
Typical Cepheid variability period: About two weeks.
Calibration with Parallax:
- Some nearby Cepheids are close enough for parallax measurements.
- Parallax measurements give us their true distance, which helps calibrate the period-luminosity relationship for all Cepheids.

The cosmic distance ladder is the concept of building upon different distance measurement techniques that overlap.
Nearby distances are measured using parallax, which overlaps with Cepheid variable measurements, and so on.

Cepheids are individual stars, so we need to be able to resolve them.
They are useful for measuring distances within our galaxy and to nearby galaxies like the Magellanic Clouds and Andromeda.
However, at cosmological distances, we can no longer resolve individual Cepheid stars.

Type Ia supernovae are used to measure distances to very distant objects.
The discovery of their use as standard candles led to the understanding that the universe is expanding at an accelerating rate (Nobel Prize).

Supernovae are the explosive deaths of stars where a star will explode and it will blow off its outer layers.
Type Ia supernovae are special because they all explode in the same way, from stars of the same mass.
When a type Ia supernova explodes, it becomes extremely bright—as bright as an entire galaxy.
Light Curve: The brightness of a type Ia supernova decreases over time, following a characteristic curve called a light curve.

Type Ia supernovae are identified by the elements present in the ejected gas from the outer layers of the star.
Because they explode in the same way, their light curves are almost identical.
Knowing their intrinsic brightness allows astronomers to calculate distances to the galaxies in which they occur using the inverse square law.

Advantage: Very luminous, allowing them to be seen at great distances.
Challenge: Relatively rare events.
Unlike Cepheids, which pulsate for millions of years, supernovae happen only once.
The timescale for the decay of a supernova's brightness is a few weeks.

Supernova 1987A was one of the nearest and most well-studied type Ia supernovae in recorded history.
It was fortunate that many telescopes were pointed in its direction at the time of the event.

Modern surveys canvas the entire sky every night, searching for supernovae.
In the past, only a few supernovae were found per year; now, several are found per night.
These surveys help understand new physics and refine distance measurements to the edges of the universe.

The Vera Rubin Observatory in Chile, with its Legacy Survey of Space and Time (LSST), will begin operations in 2024.
It will observe the entire sky every three nights, with one goal being observing supernovae.
Institutions worldwide, including those in Australia, will be involved in data analysis.

Historically, supernova discoveries were rare (e.g., one in 500 AD, another in 1600 AD).
The increase in discoveries is due to better instrumentation.