NASA began working on a special telescope in the 1970s. This telescope, called the Hubble Space Telescope (HST), was finally launched in 1990 and is still taking amazing pictures and collecting important information about space today. But why is it called the Hubble Space Telescope? It's named after an American astronomer named Edwin Hubble who made some really important discoveries in science. He noticed that the light frequency of galaxies was changing and figured out that galaxies were moving away from each other. This led to the creation of something called Hubble's Law which we'll explain more about in this article. Basically, Hubble's Law tells us how fast objects in space are moving away from us because the universe is expanding.

Hubble’s law explained

Edwin Hubble was a scientist who studied objects called nebulae. He was particularly interested in spiral-shaped ones, and there were two theories about what they were: either they were part of the Milky Way, or they were galaxies far away from us. Hubble observed the light emitted by these nebulae and noticed that the further away they were from Earth, the more the light shifted to the red part of the spectrum. By measuring this redshift in 20 spiral nebulae, he discovered that galaxies are moving away from us at velocities that are proportional to their distance. This means that galaxies that are further away from us are moving away faster than those that are closer. This is known as Hubble's Law.

To understand this law, we need to know about something called the Doppler effect. When an object emits waves (like light) and moves towards us, the waves are compressed and the light appears more blue. When it moves away from us, the waves are stretched and the light appears more red. According to measurements, the accepted value of Hubble's constant is 73.8km/s/Mpc. This velocity might seem fast to us, but it's not even 0.1% of the speed of light. This law predicts that galaxies will eventually disappear from our view. It's important not to confuse Hubble's constant with how fast galaxies move - they move at different velocities based on how far away they are. If you want to know more about the Doppler Effect check out our explanation with more info and calculations.

Hubble’s law graph

To find the relationship between the velocity of recession and the distance, Hubble analysed more than 20 galaxies. In his analysis, he plotted their distance from the Earth in parsecs against their velocity. You can see the results of this plot below:

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From this, Hubble concluded that an increasing linear relationship exists between the distance and the velocity.

A parsec (pc) is a length unit used to measure more considerable distances outside the Solar System (extrasolar distances). A parsec equals the distance covered by light in space during 3.26 years. Using this definition, Sirius, which is the brightest star in the Northern Hemisphere, is 2.64 parsecs away from Earth.

Hubble’s law equation

Hubble's Law can be represented by a simple equation: v = Ho x D. In this equation, v is the recessional velocity in km/s, Ho is the slope of the line (also known as Hubble's constant), and D is the distance from the galaxy to Earth measured in Mpc.

If you want to learn more about how astronomers are able to measure the universe's expansion rate, including using a basic tool of geometry called parallax, head over to NASA's Hubble site.

Hubble’s law equation examples

Using the equation v = H0 x d, where v is the velocity of the galaxy, H0 is the Hubble constant, and d is the distance from Earth, we can calculate the velocity of the NGC 55 galaxy if it is at a distance of two megaparsecs from Earth. To do this, we first need to convert the distance from parsecs to kilometers. A parsec is a distance covered by light in 3.26 light years, and the distance covered by light in a vacuum in one second is 300,000,000 meters. Therefore, to convert two megaparsecs to kilometers, we must multiply the distance by the number of seconds in one year (31,556,926 seconds) and by 3.26. This gives us a distance of approximately 6.12 x 10^22 meters.

Next, we need to convert the Hubble constant from km/s/Mpc to m/s/m. The Hubble constant is typically expressed in km/s/Mpc, which means for every million parsecs of distance, the velocity of a galaxy will increase by a certain amount (in kilometers per second). To convert this to meters per second per meter, we divide by 3.086 x 10^19 (the number of meters in one megaparsec). This gives us a value of approximately 2.27 x 10^-18 m/s/m.

Finally, we can substitute the values we have calculated into the Hubble's law equation to obtain the velocity of the NGC 55 galaxy. Multiplying the Hubble constant by the distance of 6.12 x 10^22 meters gives us a velocity of approximately 1.39 x 10^5 m/s.

It is important to note that almost all galaxies present a redshift in the visible light, which means they are moving away from us. However, there are some galaxies that present a blueshift instead, which is the result of gravitational forces. A blueshift is the decrease in the emitted wavelength as the emitter (the object producing the electromagnetic waves) moves closer to the observer. Androm one of these galaxies that has blueshift, which is a of the gravitational attraction between our galaxy (the Milky Way) and Andromeda. It has been predicted that both galaxies will collide after some millions of years.

Andromeda and the Milky Way have a blueshift because the dominant force between them is gravity (red arrows).

Did you know? Because Hubble’s law helps us to calculate (approximately) how much time has passed since galaxies started moving, researchers are able to gather information about the early stages of the universe and understand the Big Bang theory better.

Universe expansion

Hubble's discovery of galaxies moving further away from our local group of galaxies was a groundbreaking observation that laid the foundation for other theories on the accelerating expansion of the universe. Later evidence came from measuring the redshift of stellar objects known as supernovae. Type 1a supernovae are of particular importance in this regard, as they have a consistent intrinsic brightness that allows them to be used as standard candles for measuring cosmic distances.

Type 1a supernovae are the result of the interaction of two stars in a binary system. One of the stars is a white dwarf, while the other is a companion star that orbits closely. The white dwarf star accretes gas from its companion, and when it reaches a critical mass, it undergoes a runaway nuclear fusion reaction that results in a catastrophic explosion.

Because type 1a supernovae have a consistent intrinsic brightness, they can be used to measure cosmic distances by comparing their observed brightness to their expected brightness. By measuring the redshift of these supernovae, astronomers can also determine the rate at which the universe is expanding.

The evidence from type 1a supernovae suggests that the rate of expansion of the universe is not constant, but rather is accelerating. This has led to the development of the theory of dark energy, which posits that there is a mysterious force that is causing the expansion of the universe to accelerate. The nature of dark energy is still not well understood, but ongoing research continues to shed light on this enigmatic phenomenon.

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Because the mass at which this explosion happens is fixed, the luminosity is similar every time it happens. A near-constant luminosity can provide a constant flux of radiation (light), and researchers can use this to measure the distance of the star system. Observations of these binary systems were the ones that offered clues about the expansion of the universe (type 1a supernovae were travelling further away with time).

Hubble’s Law - Key takeaways Edwin Hubble observed a change in the light frequency of galaxies and provided critical evidence on the recessional velocity. This observation led to the formation of Hubble’s law, Hubble’s law is the observation that galaxies are moving away from us with a velocity that is proportional to their distance from us. In other words, further galaxies recede quicker than closer galaxies. Hubble concluded that an increasing linear relationship exists between the distance and the velocity. Almost all galaxies present a redshift in the visible light, but there are some that present a blueshift instead, which is the result of gravitational forces. The idea of an expanding universe was later corroborated by the study of type 1a supernovae in other galaxies.

Hubble’s Law

What is Hubble’s law?

Hubble’s law is the observation that galaxies are moving away from us with a velocity that is proportional to their distance from us. In other words, further galaxies recede quicker than closer galaxies.

How does Hubble’s law support the Big Bang theory?

Hubble’s law helps us to calculate (approximately) how much time has passed since galaxies started moving. This allows us to gather information about the early stages of the universe and understand the Big Bang theory better.

Do all galaxies follow Hubble’s law?

No, not all galaxies follow Hubble's law. Almost all galaxies present a redshift in the visible light, but there are some that don’t. These galaxies present a blueshift instead, which is the result of gravitational forces. 

How does Hubble's law depend on parallax?

Hubble researchers use trigonometric parallax to determine an object’s distance. This method detects the tiniest apparent shift in an object's location caused by a shift in the observer's viewpoint. 

How do you use Hubble's law?

We can use Hubble's law to calculate the velocity of galaxies. The equation is v=Ho·D where v is the recessional velocity measured in kilometres per second, Ho is Hubble’s constant, and D is the distance from the galaxy to Earth measured in megaparsecs.

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