Stars in our universe come in all colors and sizes. The Sun is the most familiar. The rest are just specks of twinkling light to our eyes.
While the Sun seems huge in our skies, it is, in fact, an average star when compared to others in our galaxy alone.
Neutron stars are just 12.5 miles or 20 km in diameter while supergiants are, true to their name, vast. Their radii can be 1,500 times the size of our Sun’s!
Because the Sun is the closest star to us, it is easy to measure its size. Its angular size is half a degree, which is about the same as the width of your pinkie finger held at arm’s length.
But stars are so far away that it is not possible, even for the largest telescopes, to determine their diameter as easily as we do it for the Sun. And that’s before atmospheric blurring and a telescope’s intrinsic errors that dull the measurements.
How, then, do we measure the sizes of stars? And, where can we see colossal stars in our sky?
Let’s take a look.
How Do We Measure the Size of Stars?
Quick Answer: There are three methods for measuring star sizes. Nearby stars are measured using Lunar transit times but most star sizes come from mapping their temperature and luminosity to the Hertzsprung-Russell diagram. Separately, eclipsing binaries are accurately measured as one star passes in front of another.
The Lunar Transit Method of Calculating Star Sizes
Astronomers can use the time taken for an object to pass in front of a star to measure its size. This is known as the transit method because a transit is the name given to one astronomical body passing in front of another, e.g. when Mercury transits the Sun.
The most practical way for us to use this method on Earth is when the Moon passes in front of a star. When this happens, astronomers calculate the time it takes for the star’s brightness to drop as it passes behind the Moon.
This is not as easy as it sounds. The Moon is traveling across the sky at a fair pace and the time it takes to cover a star is very short. To get accurate results, the dimming needs to be measured on a millisecond basis by an accurate camera.
Provided that the star is bright enough and the distance is known, its diameter can be calculated. Unfortunately, this only works with bright stars in our neighborhood that lie on the Moon’s path. That’s not very many, but this paper catalogs 124 stars measured by Lunar transit.
Measuring the Size of Eclipsing Binary Stars
Eclipsing binaries are a type of star that provides a similar way to calculate sizes. As their name suggests, this is a binary star where one member eclipses the other from our point of view.
Since the smaller star orbits the bigger (and usually the brighter) star, astronomers calculate the dip in brightness caused when the smaller star transits the larger star. The accurate measurement of this process lets us determine the size of the larger star.
There are four stages to this eclipse (numbers refer to the diagram):
- The point when the smaller star begins to transit (its body appears to just touch the bigger star’s disk). The larger star begins to dim at this point.
- The smaller star is completely in front of the larger star. This is when we begin timing how long it takes the smaller star to cross the disc of the larger one.
- The smaller star is on the other edge of the disk but hasn’t crossed over yet. This is when we stop the timer we began in the previous step.
- When the smaller star begins to move out of the bigger star’s disk, its luminosity begins to get back to normal again.
The size of the larger disc is calculated by multiplying the speed of the smaller star by the time taken for it to cross the larger star’s disc. I.e. the distance traveled by the small star is equal to the bigger star’s diameter, which we can work out using speed x time.
How The Size of Most Stars is Calculated
Scientists have known for a long time that there is a predictable relationship between a star’s temperature and luminosity (which measures the energy emitted in a second), and its size.
The Hertzsprung–Russell diagram, shown below, plots the relationships between temperatures and luminosities of stars and shows astronomers trends in those relationships.
This is also one of the main bases for classifying the different star types, alongside the Yerkes Luminosity classes.
The Hertzsprung-Russell diagram shows the following:
- On the X-axis, the surface temperature of the star decreases as you move from left to right, which is also shown as color.
- On the Y-axis, the luminosity increases as you move from bottom to top. Note that the Sun is given the measurement of ‘1’ and all other luminosities are with respect to that, e.g. 1,000 times more luminous (103) or 10,000 times less luminous (10-4)
- The majority of stars are located in the middle line. These are known as main-sequence stars and you can see the Sun highlighted in there.
- Stars with lower temperature and higher luminosity are in the top right corner. These stars have larger diameters, i.e. they are the giants and supergiants.
- Stars with higher temperatures and a lower luminosity are in the lower central belt. These stars have smaller diameters, such as white dwarf stars.
Although collecting luminosity and temperature data is not easy, it is something that professional astronomers are accomplished at. With those two pieces of information, the size of a star can be ascertained, and this is how we’ve measured the majority of the stars we have sizes for.
What is The Largest Star in the Milky Way?
Quick Answer: The largest known star in the sky is Stephenson 2-18, which is even bigger than former record holder UY Scuti. It has a radius that is 2,150 times that of our Sun, It is so huge, that if it were to replace our Sun, it would engulf all the planets out to and including Saturn!
- Absolute Size: Its radius is 9.9 AU (almost ten times the distance from Earth to the Sun)
- Size compared to our Sun: 2,150 times our Sun’s radius
- Age: Currently estimated to be 14 to 20 million years old
- Life expectancy: Unknown
- Constellation: Scutum
- Coordinates: R.A. 18h 39m 02s, Dec. 06° 05’ 11”
- Apparent Magnitude: 15.1 (can’t be seen with a backyard telescope)
The Largest Stars We Know Of
The Milky Way alone consists of at least 100 billion stars, and we can only see about 10% of them in a backyard telescope. Then, of course, there are numerous galaxies like Andromeda that are bigger than ours and potentially contain more stars.
Stars in other galaxies are too far away for us to measure accurately, so this section only covers stars in our own Milky Way and the Large Magellanic Cloud.
UY Scuti
With a radius of 1,708 times our Sun’s radius, UY Scuti is a red supergiant 9,500 light-years from Earth. Unlike normal stars though, UY Scuti is a variable star.
It fluctuates in not just its magnitude (from a maximum of 8.29 to a minimum of 10.56), but also its radius. Its size changes with an error margin of as much as 192 times our Sun’s radius.
We can view it as a pulsating star with a small telescope.
Its size is so huge that if it were to replace the Sun in our solar system, UY Scuti would engulf all planets until Jupiter!
- Absolute Size: 7.9 AU, although its radius fluctuates by 192 solar radii
- Size compared to our Sun: 1,708 times our Sun’s radius, vares every 740 days
- Age: Unknown, discovered in 1860
- Life expectancy: Unknown
- Constellation: Scutum
- Coordinates: Right Ascension: 18h 27m 37s, Declination: -12° 27’ 59”
- Apparent Magnitude: 9.3
WOH G64
WOH G64 is a strong contender for the ‘biggest known star’ title. It is also a variable star in the Large Magellanic Cloud, 160,000 light-years away from us. Its radius varies between 1,540 and 2575 times Sun’s radius — a size that would engulf even Saturn if the star were to replace our Sun.
Despite its size, WOH G64 is not visible with either a naked eye or an amateur telescope because it is surrounded by its own shredded material.
- Absolute Size: 11.97 AU
- Size compared to our Sun: Varies between 1,540 and 2,575 times our Sun’s radius every 800 days
- Age: Unknown. Discovered in the 1970s
- Life expectancy: Unknown
- Constellation: Dorado, or the Dolphinfish
- Coordinates: Right Ascension: 04h 55m 11s, Declination: -68° 20’ 30”
RW Cephei
Unlike the first two in the list, RW Cephei varies its brightness and size semi-regularly and fluctuates between being a yellow and a red supergiant star.
If it replaced the Sun, this star would also engulf our solar system until Jupiter.
It is visible with telescopes and binoculars, and even the naked eye if the skies are dark enough.
- Absolute Size: 7.1 AU
- Size compared to our Sun: Greater than 1,000 solar radii
- Age: 18.7 million years
- Life expectancy: About 30 million years
- Constellation: Cepheus
- Coordinates: Right Ascension: 22h 23m 07s, Declination: +55° 57’ 48”
- Apparent Magnitude: 6.5
VY Canis Majoris
One of the brightest and biggest stars in the Milky Way, Canis Majoris is a red supergiant located 4,892 light-years from Earth. Its size would engulf Jupiter if it were placed at the center of our system.
It cannot be viewed with a naked eye but is easily seen with a small telescope.
- Absolute Size: 6.6 AU
- Size compared to our Sun: 1,420 times solar radii
- Age: 8.2 million years
- Life expectancy: This is a star nearing its death. It could live for another few hundred thousand years (which is very little time, cosmologically speaking).
- Constellation: Canis Major
- Coordinates: Right Ascension: 07h 22m 59s, Declination: -25° 46’ 03”
- Apparent Magnitude: 8.9
AH Scorpii
AH Scorpii is also a semi-regular variable star with a periodicity of 714 days. Its size would swallow our solar system out to Jupiter.
This star also cannot be seen with a naked eye but is visible with a small telescope.
- Absolute Size: 6.5 AU
- Size compared to our Sun: 1,411 times solar radii
- Age: 2.9 billion years old
- Life expectancy: Unknown
- Constellation: Scorpius
- Coordinates: Right Ascension: 17h 11m 17s, Declination: -32° 19’ 30”
- Apparent Magnitude: 8.2
S Persei
Another red supergiant, S Persei is so huge that it would gobble up everything in our system until Jupiter.
Needs a telescope to be viewed.
- Absolute Size: 5.6 AU
- Size compared to our Sun: Over 700 times solar radii
- Age: Unknown, although it is part of the younger population
- Life expectancy: Unknown
- Constellation: Perseus
- Coordinates: Right Ascension: 2h 22m 52s, Declination: 58° 31′ 11″
- Apparent Magnitude: 9.5
Westerlund 1-26
A red supergiant that lives on the borders of a star cluster. Large enough to engulf our system until Jupiter.
This star is too faint for a backyard telescope.
- Absolute Size: 5.42 to 5.68 AU
- Size compared to our Sun: 1165 to 1221 times solar radii
- Age: 3.5 million years
- Life expectancy: Unknown
- Constellation: Ara
- Coordinates: Right Ascension: 16h 47m 05s, Declination: -45° 50’ 37”
- Apparent Magnitude: 17.3
V358 Cassiopeiae
This red star is 11,934 light-years away from us, and is easily seen in an amateur telescope.
- Absolute Size: 0.94 AU
- Size compared to our Sun: 203 times the solar radius
- Age: Unknown
- Life expectancy: Unknown
- Constellation: Cassiopeia
- Coordinates: Right Ascension: 23h 30m 28s, Declination +57° 58’ 34”
- Apparent Magnitude: 9.5
Betelgeuse
A fascinating and famous double star in the constellation of Orion that’s at the end of its life. It is easily visible to the naked eye.
- Absolute Size: 4.12 AU
- Size compared to our Sun: 887 times the solar radius
- Age: 9 to 10 million years old
- Life expectancy: Also 10 million years; Betelgeuse is nearing its end
- Constellation: Orion
- Coordinates: Right Ascension: 05h 55m 11s, Declination +07° 24’ 25”
- Apparent Magnitude: 0.6
KY Cygni
Also a red supergiant in Cygnus so large that it would engulf our system until Saturn if it were to replace the Sun.
KY Cygni can be seen in a decent backyard telescope.
- Absolute Size: 1.1 AU
- Size compared to our Sun: Over 1000 times the solar radius
- Age: 942 million years
- Life expectancy: Unknown
- Constellation: Cygnus
- Coordinates: Right Ascension: 20h 25m 58s, Declination +38° 21’ 07”
- Apparent Magnitude: 10.9
Summary
Our universe is rich with stars of all sizes and lifespans. Astronomers mainly measure their sizes using luminosity and temperature, which are physically related.
As we learned, some of them are visible through amateur telescopes. With all the details mentioned above, would you like to take a shot at observing them?
