Mira Makes January Nights “Wonderful”

Mira, an easy-to-observe pulsating variable star, reaches peak brightness this month. Don't be shy, come look her in the eye. 

Hot potato

A visible-light image of Mira taken by the Hubble Space Telescope reveals the star's odd shape. Its resemblance to a football may be due to changes in the star's shape during its expansion-contraction cycles or to unresolved starspots on its surface. Mira is 700 times larger than the Sun.
NASA / ESA

Mira, where have you been all these months? The sky just isn't complete without your candlelight glow. Much of the year, this famed variable star goes missing, too faint to see without binoculars or a small telescope. Then, in the span of just a few weeks, it leaps from invisible to obvious with startling suddenness.

In the first week of December 2017 Mira was undetectable with the naked eye from my observing spot. But just two weeks later, the star shone at magnitude 4.5. Now, in January, it's a ripe peach of magnitude 3.5. Based on brightness estimates submitted to the American Association of Variable Star Observers (AAVSO), Mira is currently at peak light.

The star's light varies cyclically over 333 days (~11 months) from about magnitude 10 at minimum to 3rd magnitude at maximum. Not all maxima are equal, though. At times Mira outdoes itself as it did in 2011, when it rivaled Polaris in brightness.

Follow the Hyades arrow to Mira

Despite its location in a sprawling, faint constellation, Mira is easy to find. Use the nearby Hyades and Pleiades as an arrow to point to Alpha (α) Ceti (Menkar), then wiggle southwest 12° to Mira. It's currently one magnitude fainter than Menkar. Mira's size and surface temperature vary about 20% from maximum to minimum light.
Stellarium with additions by the author

This season, it's closer to its historical maximum and easy to spot in the head of Cetus, the Whale. The Hyades point to 2nd-magnitude Menkar with Mira a little more than a fist farther to the southwest.

Mira is the prototype of the Mira-type variables, red giant stars transitioning from an active life of burning nuclear fuel to falling asleep in front of the TV, figuratively speaking. If we could peer deep inside Mira, we'd find a inert core of carbon and oxygen "ash" leftover from the fusion of helium. Miras aren't massive enough to fuse their ash into more complex elements, so the C-O core contracts under the force of gravity and ultimately evolves into a hot, superdense white dwarf.

Ups and downs of a star

Mira's changing brightness, shown in this light curve from 2011 through mid-January 2018, graphically depicts the star's regular pulsations. Notice that both peak and minimum brightness varies cycle to cycle. The missing parts are when Mira was only up in the daytime or in bright twilight, when observations weren't possible.
AAVSO

Meanwhile, helium fusion fires up in a shell just outside the core and hydrogen fusion in another shell around that one like a layered chocolate ball. The outpouring of heat from all this internal activity causes the star's outer layers to expand outward, cool, and then fall back toward the core, where they're reheated and a new cycle of expansion begins. Mira's steady rhythm resembles the relaxed breathing of someone in deep sleep. Very deep sleep — one inhale, an exhale, and a second inhale take all of 331 days. Other Mira-type stars "breathe" or pulsate with periods that range from about 80 to 1,000 days.

The details of the pulsation mechanism get technical but they're basically caused by the changing opacity — the ability of a matter to obstruct the flow of energy by absorption —  of ionized hydrogen in a layer below the star's surface. The hydrogen acts as a valve that traps and releases the star's internal heat with each pulse. More information can be found here.

Mira comparison-star magnitude chart

Mira is an easy target for binoculars and sometimes the naked eye. Comparison-star magnitudes, courtesy the American Association of Variable Star Observers, are given to the nearest tenth with the decimal points omitted to avoid confusion with faint stars. Alpha (α) Ceti (Menkar) is also a pulsating red giant, but its brightness varies by only a tenth of a magnitude. Click on the chart for a larger image.
S&T

Pulsations cause a change in Mira's light that generally follow a regular pattern: a sharp rise to maximum followed by a slower decline to minimum. Mira-type variables are brightest around the time they're smallest and faintest when most distended. You can estimate Mira's brightness with the chart above or this one. The star will fade through winter but remain visible to the naked eye or with binoculars up to solar conjunction. When it returns to view in the morning sky in mid-July, Mira will still be fading toward its September minimum. It will leap back to max in December. Since its period is about one month shy of a year, the time of maximum slips backward about a month each year. In 2020, maximum will occur in November.

Pulsations and strong stellar winds can blow off significant amounts of Mira's atmosphere. Get a good look at the star now before its final act. Sometime in the next few thousand years, it will shed its billowy envelope like a monarch leaving the chrysalis. The now Earth-sized, white-dwarf core will be exposed to view, its matter so dense a teaspoon of it will weigh as much as a Chevy Suburban. White dwarfs sizzle around 180,000°F (100,000°C), making them fantastic emitters of UV light. It's this light that will ultimately transform the star's fleeing atmosphere into a fluorescent butterfly. On that distant day, Mira will become a planetary nebula.

Banded beauty

Mira's spectrum, recorded here in February 2017, displays multiple molecular bands, many of which are due to absorption by titanium oxide in the star's atmosphere.
Rob Kaufman

If you have a spectroscope, Mira displays a beautiful spectrum with lots of absorption lines. I use one from Rainbow Optics on my 10-inch scope. Stars like Sirius show a rainbow spectrum crossed by several thin, dark lines, each representing an energy state of the hydrogen atoms in the star's atmosphere. Mira is a much cooler star, cool enough for molecules to form in its atmosphere. As they absorb the light radiating from the brighter photosphere, they show as prominent dark bands in its spectrum. The most obvious absorption comes from titanium oxide, shuttering the spectrum like a set of Venetian blinds. With Mira near maximum light, this is the best time to view (or photograph) these amazing bands. Titanium oxide is excellent at absorbing sunlight, too, the reason you'll find it used in sunscreens!

Wonderful sight

The author captured this image of Mira on January 13th, when the star was at magnitude 3.5.
Bob King

Mira was first recorded as a nova by Dutch amateur astronomer David Fabricius in August 1596. He witnessed its reappearance in February 1609, but apparently didn't study it further. Johann Holwarda rediscovered the star in 1638 and determined its period to be 11 months. A year later, Johannes Hevelius got his first look and decided on the name Mira, Latin for "The Wonderful."

For more than 420 years, generations of amateur and professionals astronomers have grooved to its rhythm. You can, too.

29 thoughts on “Mira Makes January Nights “Wonderful”

  1. Joe StieberJoe Stieber

    I was out for a look at Mira this past weekend, January 13 & 14, 2018, following-up on a Sue French article about variable stars back in October 2017 that indicated Mira should be peaking in January 2018. Indeed, it was visible with unaided eyes from my suburban backyard. It’s so fascinating to see a relatively bright star where none had been a few months before. I use a different approach to finding Mira. I start at Aries, which points to Eta Psc, then go down the eastern rope of Pisces to Alrischa, the knot in the ropes, then hop 7 deg to Mira. BTW, if you have your binoculars, or a scope, Uranus is only 10 deg from Alrischa now, so take a look at it too (or perhaps even unaided eyes if you’re at a dark site).

    P.S. — Another great article! Thanks Bob.

    1. Bob KingBob King Post author

      Thank you, Joe! And thanks for the Uranus tip. I totally agree about the many paths to Mira. I’ve done several finder maps over the years, and I swear each one is different.

    2. Anthony BarreiroAnthony Barreiro

      Sue French’s article inspired me to start looking at variable stars more consistently, through binoculars from my urban back yard. Chi Cygni was fun to watch brighten and fade in the autumn. I’ve been enjoying Mira too. I also use Aries to Pisces as my star-hop, with a rest stop at Uranus. My limiting visual magnitude is about 3, so all I can see of Cetus naked-eye is Alpha and Beta Ceti. Last week I almost convinced myself I could see Mira naked-eye, but I’m not sure. Now we’ve got a series of welcome rain storms, so I’ll have to wait for a break in the weather to look again.

      I don’t understand why Mira-type variables get dimmer as they get bigger. Normal red giants get brighter as they get bigger. I should read the articles you link to above, Bob. But I don’t have time to get enlightened right now.

        1. Bob KingBob King Post author

          Thanks for keeping us in touch. We’ve been cloudy here for the past few nights. Also, I bet you were happy to see how the meteorites were being handled.

  2. Walter ClaytonWalter Clayton

    I’ve been waiting for Mira to flare up this past year and noted that I could see it from our dark-sky site back in December. Since then, I have been trying to catch a peek at Mira every clear night.

    One thing I have noticed is that is does fluctuate! I have been using the stars in Cetus’ neck (Alpha, Gamma and Delta) and Alpha Piscium as comparison stars. Some nights it has been as bright as Gamma Cet, some as bright as Delta, but, never as bright as Alpha Cet.

    I did lose a few days due to the recent “snow storm” we had here in South Georgia, but, last time I checked, Mira was still holding it’s own.

    I was never a Variable star guy, but, I still found this interesting.

    Thanks!

    Walter Clayton

    1. Bob KingBob King Post author

      Thanks for writing, Clayton and sharing your view of our featured star. I was never a variable guy years ago, but I got hooked because I liked seeing things change so quickly in the sky. Submitting observations of variables to the AAVSO was also a way to make a small contribution to the science of variable star behavior.

      1. Walter ClaytonWalter Clayton

        Just came in from checking tonight (first nice clear night we’ve had in a few days), definitely dimmer than Gamma, but, brighter than Delta.

        I’ve thought about doing some “scientific” observations, instead of just viewing, but, haven’t pull the trigger. Maybe one day.

        WAC

  3. Tom-Reiland

    I just took a look at Mira from my upstairs windows. I have little desire to go out in single digit temps. It certainly stands out in that area of the sky as viewed with 10 X 50 Binocs and nude-eye. It’s one of my favorite variable stars, though I’m not into them like my mentor and friend George Lindbloom was. He did give me an appreciation of variables and the thrill of watching for the rise and fall of their brightness. Chi Cygni, Algol and many of the red, carbon stars such as R Leporis, T Lyrae and WZ Cassiopeiae are the ones that I enjoy observing.

      1. Tom-Reiland

        I forgot to mention that I estimated Mira last night at 3.4 magnitude using Gamma Ceti as a comparison star at 3.6 mag. Mira was slightly brighter than Gamma. I could be off by 0.1 mag at the most, putting it at least at 3.5 mag.

  4. paul-livio

    Bob:
    Excellent article. I have a Rainbow Optics (now discontinued) spectroscope but I found that the Rigel Systems spectroscope works better. Check it out. We use it at CMC for the astronomy lab.
    Regards,

    Paul Livio

  5. Raymond

    Hi Bob, thanks for great article. I’ve been following Mira’s rise for a couple of months now, as a new observer on AAVSO (code HRBA). So they call it a “pulsating” variable, okay. But the clock-like regularity makes me suspect that an unseen close companion could be involved as well. Maybe something that gets very close every 332 days, pulling luminous gases off the red giant, making it look like a football? Think that’s a possibility?

    1. Raymond

      Just following up on my question… You say, “Mira’s steady rhythm resembles the relaxed breathing of someone in deep sleep.” Okay, but we don’t breathe with clock-like regularity. What causes Mira to breathe on such a precise schedule?

      1. Bob KingBob King Post author

        Raymond,
        I wouldn’t describe Mira as clock-like. It does have a regular rhythm but it varies a bit as does the star’s peak and least brightness. But yes, its cycle is predictable and regular. As Mira’s outer layer contracts and compresses, it checks the flow of heat from the star’s interior. Heat then builds up — along with an increase in outward pressure — that pushes the outer layer back out. We see the star expand and fade. The outer layer then cools and gravity pulls it back in to begin a new cycle of expansion.

    2. Bob KingBob King Post author

      Hi Raymond,
      The possibility you mention is very real for a couple other classes of variable stars but not in Mira’s case. Mira, like its cohorts and also the Cepheid variables, pulsates on its own without any help from a companion. The pulsations are tied to the star’s internal workings. That said, Mira does have a close white dwarf companion that interacts with material Mira blows off in stellar winds. Algol-type variable stars (Algol, a.k.a. the Demon Star in Perseus) also fade and brighten with regular cycles that much more like clockwork than Mira’s. The cause? They’re eclipsing binaries where a companion star blocks part of all of the primary star’s light every time it orbits in front of it.

      1. Raymond

        Thanks Bob. Just seems so odd that internal pulsations can be predicted with such precision. For example, the SEDS site gives the period as 331.96 days, which they use to predict precise maxima all the way out to 2023: Jun 13, 15:50. That looks clock-like to me. Or are they exaggerating the precision? I wonder how can we know that there isn’t a close companion in a highly elliptical orbit…

        1. Bob KingBob King Post author

          Raymond,
          That precision might be an average — Miras can vary some over their cycles. We know of many thousands of Mira-type variables, semi-regular variables and Cepheids, and the pulsation mechanism best fits the data and theoretical models. If most had companions, it would seem likely we’d have detected them by now (some do, as Mira does, but it’s not the cause of the pulsations). That said, some scientists take issue with the regularity of Mira periods noting that the stars’ aspherical shapes preclude regular pulsation. They posit close-orbiting companions as the cause. But for now, the consensus in the astronomical community is pulsation.

          1. Raymond

            Interesting, thanks Bob. I looked at Mira over the weekend, interesting that the peak seems to be hanging on. The light curve looks pretty flat over the past three weeks.

          2. Bob KingBob King Post author

            Raymond,

            It sure is hanging in there, isn’t it? Mira stars typically head “down the curve” slower than they climb up it. Again, thanks for the question that started this discussion. Always something interesting to be learned.

  6. Raymond

    You’re welcome. I did find a 2002 paper online by P. Berlioz-Arthaud that argues for the close companion hypothesis. They even thought their idea could apply to Cepheids as well. Just shows to go ya, we can’t get too definitive about some things…

          1. Raymond

            Yep, still bright enough to see naked-eye from my urban rooftop last night. Maybe time to point the Galaxy Evolution Explorer at it again?

          2. Jakob

            Still bright tonight, 3.7. I can’t remember such behavior of this star before. I have been observing the skies för 42 years and this maxima is realy special! Jakob

  7. Bob KingBob King Post author

    Jakob,

    It’ll be interesting to see the light curve’s appearance (slope) from the start of winter through spring. I’m happy for what appears to be an extended maximum because it gives more people a chance to see and follow this wonderful star.

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