ClassX Video Lessons Light seconds, light years, light centuries: How to measure extreme distances – Yuan-Sen Ting
Light is the fastest thing we know, and because of its incredible speed, we use it to measure vast distances in space. In one year, light travels about 6 trillion miles, which we call a light year. To put this into perspective, the Moon, which took the Apollo astronauts four days to reach, is only one light-second away from Earth. The nearest star beyond our Sun, Proxima Centauri, is 4.24 light years away. Our Milky Way galaxy spans about 100,000 light years, and the nearest galaxy, Andromeda, is approximately 2.5 million light years away. Space is unimaginably vast!
But how do we know how far away stars and galaxies are? When we look at the night sky, it appears flat and two-dimensional. If you point at a star, you can’t immediately tell how far away it is. So, how do astrophysicists figure it out?
For nearby objects, we use a method called trigonometric parallax. Here’s a simple experiment: stick out your thumb and close one eye. Now switch eyes. It seems like your thumb has moved, while the background stays the same. This concept applies to stars, but since they are much farther away than your thumb, we need a bigger baseline than just the width of your eyes.
We use Earth’s orbit around the Sun as our baseline. By observing a star’s position in the sky at different times of the year, we can see a slight shift against the background of more distant stars. This method works for stars within a few thousand light years. Beyond that, the shift is too small to detect, even with our best instruments.
For greater distances, we rely on standard candles. These are objects with a known brightness, or luminosity. Imagine you have a light bulb of known brightness. As your friend walks away with it, the light appears dimmer. By comparing the observed brightness to the known brightness, you can calculate the distance.
In astronomy, a type of star called a Cepheid variable acts as our light bulb. These stars expand and contract, causing their brightness to change. By measuring the period of this cycle, we can determine their luminosity. Comparing this to the light we observe, we can calculate their distance.
However, Cepheid variables only help us measure distances up to about 40 million light years. Beyond that, stars become too blurry to resolve. Fortunately, we have another tool: Type Ia supernovae. These are massive stellar explosions that outshine entire galaxies. Because their brightness fades at a predictable rate, we can use them as standard candles to measure distances up to several billion light years.
Why is it important to observe such distant objects? Remember, light travels fast. The light from the Sun takes eight minutes to reach us, meaning we see the Sun as it was eight minutes ago. When you look at the Big Dipper, you’re seeing it as it was 80 years ago. Distant galaxies are millions of light years away, so we see them as they were millions of years ago. In this way, the universe acts as a time machine, allowing us to look back in time.
Astrophysicists study this ancient light to understand the history of the universe and our place within it. The universe constantly sends us information in the form of light, and it’s up to us to decode it.
Using everyday materials, create a scale model of the solar system to understand the vast distances between celestial bodies. Calculate the distances using light seconds and light years, and represent them proportionally in your model. This will help you visualize the concept of light as a measurement tool for space distances.
Conduct a parallax experiment using simple tools like a ruler and a pencil. Measure the apparent shift of the pencil against a distant background from two different positions. This will give you a hands-on understanding of how trigonometric parallax helps astronomers measure the distance to nearby stars.
Simulate the concept of standard candles by using a light bulb and a lux meter app on your phone. Measure the brightness of the bulb at different distances and calculate the distance based on the observed brightness. This will help you grasp how astronomers use Cepheid variables and Type Ia supernovae to measure cosmic distances.
Research and create a timeline of celestial events that have occurred at various distances from Earth. Calculate how long ago these events happened based on their distance in light years. This will illustrate how observing distant objects allows us to look back in time and understand the universe’s history.
Take a virtual tour of an observatory or use online astronomy software to explore distant galaxies and stars. Identify objects like Cepheid variables and supernovae, and learn how astronomers use these observations to measure distances. This will provide a practical understanding of the tools and methods used in astrophysics.
Here’s a sanitized version of the provided YouTube transcript:
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Light is the fastest thing we know. It’s so fast that we measure enormous distances by how long it takes for light to travel them. In one year, light travels about 6 trillion miles, a distance we call one light year. To give you an idea of just how far this is, the Moon, which took the Apollo astronauts four days to reach, is only one light-second from Earth. Meanwhile, the nearest star beyond our own Sun is Proxima Centauri, 4.24 light years away. Our Milky Way is about 100,000 light years across. The nearest galaxy to our own, Andromeda, is about 2.5 million light years away. Space is incredibly vast.
But how do we know how far away stars and galaxies are? When we look at the sky, we have a flat, two-dimensional view. If you point your finger to one star, you can’t tell how far away it is. So how do astrophysicists figure that out? For objects that are very close by, we can use a concept called trigonometric parallax. The idea is simple. Let’s do an experiment: stick out your thumb and close one eye. Now, open that eye and close the other. It will look like your thumb has moved, while more distant background objects remain in place. The same concept applies when we look at the stars, but distant stars are much farther away than the length of your arm, and the Earth isn’t very large. So, even if you had different telescopes across the equator, you wouldn’t see much of a shift in position. Instead, we look at the change in the star’s apparent location over six months, the halfway point of the Earth’s yearlong orbit around the Sun. When we measure the relative positions of the stars in summer and then again in winter, it’s like looking with the other eye. Nearby stars seem to have moved against the background of the more distant stars and galaxies. However, this method only works for objects no more than a few thousand light years away. Beyond our own galaxy, the distances are so great that the parallax is too small to detect with even our most sensitive instruments.
At this point, we have to rely on a different method using indicators we call standard candles. Standard candles are objects whose intrinsic brightness, or luminosity, we know very well. For example, if you know how bright your light bulb is and you ask a friend to hold it and walk away from you, you know that the amount of light you receive will decrease by the distance squared. By comparing the amount of light you receive to the intrinsic brightness of the light bulb, you can then tell how far away your friend is. In astronomy, our light bulb turns out to be a special type of star called a Cepheid variable. These stars are internally unstable, like a constantly inflating and deflating balloon. Because the expansion and contraction causes their brightness to vary, we can calculate their luminosity by measuring the period of this cycle, with more luminous stars changing more slowly. By comparing the light we observe from these stars to the intrinsic brightness we’ve calculated, we can determine how far away they are.
Unfortunately, this is still not the end of the story. We can only observe individual stars up to about 40 million light years away, after which they become too blurry to resolve. Luckily, we have another type of standard candle: the famous type Ia supernova. Supernovae, giant stellar explosions, are one of the ways that stars die. These explosions are so bright that they outshine the galaxies where they occur. Even when we can’t see individual stars in a galaxy, we can still see supernovae when they happen. Type Ia supernovae are usable as standard candles because intrinsically bright ones fade slower than fainter ones. Through our understanding of the relationship between brightness and decline rate, we can use these supernovae to probe distances up to several billion light years away.
But why is it important to see such distant objects? Remember how fast light travels. For example, the light emitted by the Sun takes eight minutes to reach us, which means that the light we see now is a picture of the Sun eight minutes ago. When you look at the Big Dipper, you’re seeing what it looked like 80 years ago. Those distant galaxies? They’re millions of light years away, and it has taken millions of years for that light to reach us. The universe itself is, in some sense, an inbuilt time machine. The further we can look back, the younger the universe we are probing. Astrophysicists try to read the history of the universe and understand how and where we come from. The universe is constantly sending us information in the form of light. All that remains is for us to decode it.
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This version maintains the original content while ensuring clarity and readability.
Light – Electromagnetic radiation that is visible to the human eye and is responsible for the sense of sight. – The speed of light in a vacuum is approximately 299,792 kilometers per second.
Distance – The amount of space between two points, often measured in units such as meters or light-years in astronomy. – Astronomers use the light-year as a unit to express the distance between stars.
Stars – Massive, luminous spheres of plasma held together by gravity, often found in galaxies. – The Sun is the closest star to Earth and provides the energy necessary for life.
Galaxies – Large systems of stars, stellar remnants, interstellar gas, dust, and dark matter, bound together by gravity. – The Milky Way is the galaxy that contains our Solar System.
Brightness – The perceived intensity of light from a celestial object, as seen from Earth. – The brightness of a star can be affected by its distance from Earth and its intrinsic luminosity.
Luminosity – The total amount of energy emitted by a star, galaxy, or other astronomical object per unit of time. – A star’s luminosity is a key factor in determining its classification and life cycle.
Supernovae – Explosive events that occur at the end of a star’s life cycle, resulting in a sudden increase in brightness. – Supernovae can outshine entire galaxies and are crucial for dispersing elements into space.
Parallax – The apparent shift in position of a nearby star against the background of distant objects, used to measure stellar distances. – By observing the parallax of a star, astronomers can calculate its distance from Earth.
Universe – The totality of space, time, matter, and energy that exists, including all galaxies, stars, and planets. – The Big Bang theory describes the origin and expansion of the universe.
Astrophysicists – Scientists who study the physical properties and processes of celestial objects and phenomena. – Astrophysicists use telescopes and theoretical models to understand the behavior of stars and galaxies.
