Brightest Stars

See Mira the Wonderful at its brightest

Star chart showing the constellation Cetus the Whale. An arrow points to Mira's place in the sky.
In August, look east in the wee hours after midnight, or southward by dawn. Mira is part of the constellation Cetus the Whale, or Sea Monster. It’s the friendliest-looking monster you’ll ever see. In a dark sky, look for the lopsided pentagon that makes up the Whale’s Head. Will you see Mira? Only if the star is near its maximum brightness. In 2021, that’ll happen on or around August 18. Check Stellarium for a view from your location.

Mira the Wonderful

Although stars appear to shine at a constant brilliance, many are variable stars. They brighten and dim over many different timescales. Their changes in brightness are often too small to be perceptible to the unaided eye. But the star Mira, aka Omicron Ceti, is different. Its brightness changes are large and distinctly noticeable to the eye. Depending on when you look for Mira, this reddish star in the constellation Cetus the Whale might or might not be visible. It goes through its bright-to-faint-to-bright cycle about every 332 days. Mira has a predicted brightness peak coming up. It should be brightest on or near August 18, 2021. If you’d like to see this unusual star in 2021, now’s your chance.

Early astronomers noticed this star’s dramatic and regular changes in brightness. Mira sparkles in the sky, getting progressively dimmer, and a few months later, it’s gone! Then, after some months, it’s back again. Its brightness changes led the 17th century astronomer Johannes Hevelius to name the star Mira, from the Latin word for wonderful or astonishing.

So Mira is on track to hit another brightness peak around August 18, 2021. How bright will it get? That’s a question many variable star observers are eagerly waiting to find out.

Now you see it, now you don’t

Mira has an average peak brightness of magnitude 3.5. It’s not one of the sky’s brightest stars, even when brightest. It gradually fades to around magnitude 9 (too faint to see with the eye; for reference, in a dark sky, the unaided eye can barely detect a magnitude 6 star). Then it rebounds back to its peak brightness. So Mira undergoes about a 159-fold change in brightness, as it moves through its 332-day brightness cycle.

It’s impossible to predict exactly how bright or faint Mira will become at each maximum. Have a look at the graph below, called a light curve. Mira-watchers contribute their observations to the American Association of Variable Star Observers (AAVSO). The AAVSO creates an ongoing light curve for Mira, using its Light Curve Generator tool. The light curve below covers the last 10 years. Parts of the plot with no data were when Mira was close to or behind the sun. In 2011 and 2019, Mira was as bright as magnitude 2. That’s almost as bright as Polaris, the North Star, not the sky’s brightest star, but a respectably bright star.

A plot showing up-and-down rhythmical waves of data points, brightness vs. time, that illustrate how Mira’s brightness changes with time.
View larger. | This graph shows how Mira’s brightness has changed over the past 10 years. It plots the brightness of Mira vs. time, as measured by variable star observers. Notice its greatest and least brightness varies slightly from cycle to cycle. For instance, Mira was almost at magnitude 2 in 2011 and 2019. In 2017, it hit a low of 10.5 magnitudes. Image generated by the AAVSO Light Curve Generator tool.

How to see Mira.

Catch Mira while it’s at its brightest! Then watch it as it fades away. Its peak brightness for 2021 comes in August. At that time, the constellation Cetus the Whale will be in the highest in the sky around 4 a.m. But Cetus isn’t a prominent constellation. It’s faint. You’ll want a dark sky. If you have a dark sky, you can pick out the Whale’s lopsided pentagon of a Head.

And look at the chart below. Notice that the distinctive nearby V-shaped Hyades star cluster in Taurus the Bull points to Cetus and its star Mira.

Here is a list of upcoming predicted maximum brightnesses for Mira, via SEDS:

2021: August 18
2022: July 16
2023: June 13
2024: May 10

Look for Mira around these dates in the coming years! That’s when, according to predictions, it should be at its brightest.

A star chart with stars in white against a black background. The constellations Taurus, Orion, and Cetus are labeled, as are the Hyades star cluster, Pleiades star cluster, as well as stars Menkar, Betelgeuse, and Rigel. Blue lines connect the stars for each constellation.
View larger. | Cetus is a faint constellation, and Mira isn’t super bright, even when brightest. Look for them in a dark sky. In this star chart, the V-shaped Hyades star cluster points the way to Mira. Note that Mira might or might not be the brightest star in Cetus. That’s usually the star Menkar, but … who knows? We won’t know for sure how bright Mira will get until its maximum brightness around August 18, 2021. Star chart via Stellarium.
Annotated image showing the outlines of the constellations, labels for planets and satellite trails, against a starry night landscape.
Astrophotographer Alan Dyer captured what he described as a “busy sky” on October 15, 2020. Mars was just past its opposition. Mira had just reached its peak brightness, shining at magnitude 3.4 in this image. He even caught Uranus and Neptune! Click here to view a larger version of the image. Image via Alan Dyer / AmazingSky.com / Flickr, used with permission.
A detailed star chart of the constellation Cetus, with stars marked in black against a white background, and green lines connection the stars in Cetus.
Stars in the constellation Cetus, including Mira. Image via IAU / Sky & Telescope / Wikimedia Commons.

Mira science

Early astronomers marveled at Mira’s brightness changes and considered them a great mystery. But modern astronomers know Mira as a red giant star. It’s slightly more massive than our sun but at least 330 times larger in size. Its huge surface area makes it more than 8,000 times more luminous. Mira is some 300 light-years away. It’s thought to be around 6 billion years old. Mira has a faint white dwarf companion star.

There are many types of pulsating variable stars known today. But Mira was the first of its type discovered. And so astronomers named an entire class of variable stars after it. Mira variables are stars that have one to a few times the mass of our sun. They’re near the end of their stellar lifetime, at the red giant stage. Mira variables have pulsation periods from 80 to 1,000 days, brightness variations from 2.5 to 10 visual magnitudes, and tend to shed material from their outer layers.

So Mira’s brightness changes aren’t due, for example, to some external factor (such as a disk around the star). They’re are caused by the actual expansion and contraction of the entire star, every 332 days. This expansion-contraction oscillation is a complex phenomenon related to changes in the rate that radiation escapes from the star.

Mira’s story is of special interest since our sun will someday follow the same stellar evolutionary path. About 5 billion years from now, our sun will become a Mira variable.

Closeup of two red stars, side by side.
Mira (on the right) and its companion, imaged in ultraviolet wavelengths by the Hubble Space Telescope, in 1995. The two stars are separated by 0.6 arcseconds, about 70 times the distance between the Earth and sun. Image via NASA / STScI.

Why Mira’s brightness changes

For much of its existence, Mira converted hydrogen to helium at its core as a main sequence star. When that fuel was exhausted, its core contracted, causing it to heat up. That heating triggered a new round of hydrogen-to-helium nuclear fusion in a shell around the core, causing Mira to balloon in size into a red giant star. Meanwhile, the collapsing core continued to heat up until it became hot enough for the fusion of helium to carbon, and some oxygen.

Mira is currently at a stage in its stellar evolution called the asymptotic giant branch. Its core of carbon and oxygen is inert. However, the star is still actively “burning” a layer of helium around the core, converting it to carbon. And just outside it, a shell of hydrogen is being converted to helium.

The outer layers of Mira are weakly held by gravity and are starting to waft away. Mira will eventually shed its material to form a planetary nebula, with its exposed hot core — a white dwarf star — left behind.

Mira’s 13-light-year-long tail

In 2006, the Galaxy Evolution Explorer telescope obtained ultraviolet images of Mira that surprised scientists. They revealed a long comet-like tail of material trailing the star as it sped through ambient galactic gas. Mira moves through this space at about 290,000 miles per hour (130 km/s). The tail, about 13 light-years long, is composed of gases and dust released by Mira over the last 30,000 years. The amount of gases and dust in Mira’s tail equal about 3,000 times the Earth’s mass.

Starry field in blue shades with a comet like feature across, the star Mira to the right and a tail of gas and dust stretching out to the left.
View larger. | NASA’s Galaxy Evolution Explorer telescope acquired this image of Mira in 2006. Captured in ultraviolet wavelengths, the image shows a long tail of gas and dust shed by Mira. The tail is some 13 light-years in length! That’s about 3 times the distance between our sun and the next-nearest stars. Mira itself is hidden from view in this image, in the clump of gas at the extreme right. Image via NASA / JPL-Caltech.

Mira in history.

Did the earliest stargazers notice Mira as it appeared disappeared and reappeared? If they did, they left no records of this star. The star’s earliest known history begins only 400 years ago, when Dutch astronomer David Fabricius first noticed Mira. That was in the year 1596. He assumed Mira was a nova because, as novae do, the star faded away after a few months. However, Fabricus relocated the star 13 years later. It must have surprised him!

Another Dutch astronomer, Johannes Holwarda, was the first to identify Mira as a variable star, and determined a period of 11 months. That value was refined in 1667 by French astronomer Ismael Bouillaud to 333 days, very close to the currently accepted value of 332 days.

Mira got its name, meaning wonderful or astonishing in Latin, from Johannes Hevelius in 1642.

The position of Mira is RA: 02h 19m 21s, Dec: -02° 58′ 39″.

A black and white image of several faint stars with one prominent bright star, Mira, in the center.
Mira, as imaged by the 1.24-meter UK Schmidt telescope in Australia. Image via DSS / Mikulski Archive.

Bottom line: Mira is a variable star that undergoes periodic changes in brightness every 332 days, ranging from a maximum brightness of around 3.5 visual magnitudes to a minimum brightness of about 9 magnitudes.

Read more: Mira Revisited, from the AAVSO

Posted 
August 3, 2021
 in 
Brightest Stars

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