Category: Astronomy

Exploring the cosmos

Waiting for T Coronae Borealis

Things change in the sky. Contrary to the stenuous assertions of some, the night sky is not constant, it changes. Stars move, sometimes fairly rapidly, stars fade, and sometimes brighten dramatically.

Then there are novae, stars the flare to a brilliance far beyond their normal lustre. Such a star is T Coronae Borealis, or T CrB for short. This star is a recurrent nova, a star that flares to brilliance once or twice every century.

Sometimes called the Blaze Star, T CrB is normally a dim 10th magnitude star, a star that requires a small telescope to view, lost in a field filled with similar stars. A few times in the annals of astronomy the star has blazed to second magnitude, about 1500 times brighter. In 1866 and 1946 the star rivaled nearby Alphecca, the brightest star in the constellation of Corona Borealis. It may also have been observed in 1217 and again in 1787 giving a rough period of about 80 years.

T CrB is a white dwarf that is stealing material from a stellar companion, a red giant near the end of its own fuel. When that material builds up enough the white dwarf flares into temporary brilliance as a fusion reaction tears across the surface of the stellar remnant. The cycle repeats over the centuries causing these regular novae. Eventually the accumulating mass will be too great and instead of a recurring nova the star will meet its final end as a type Ia supernova.

I am of course among those awaiting the eventual nova. Yesterday evening I took a few images of the field to capture the scene. Hopefully I can take the images again to get a before and during image of T CrB.

Last year T CrB started exhibiting behavoirs similar to what had been measured just prior to the 1946 eruption. As a result we expect the star to go nova on schedule this year, most likely in the next month.

We have been waiting, stargazers keeping an eye on the constellation all summer.

Not yet.

Awaiting the recurring nova of T CrB
T CrB on the evening of 22 Aug 2024… Still about 10th magnitude, no nova yet. The 4th magnitude star Epsilon CrB at the top.

A Better Sun…

After yesterday’s unsatisfactory image of the Sun I set out to do it right today.

The conditions were about the same, the telescope and camera the same, but no clouds cutting short my time at the camera. I took a few video segments and processed these with AutoStakkert! to produce a stack of the best 900 frames out of 1800. The result is a much better image.

The image is more representative of what you see at the eyepiece, with somewhat more detail visible to the eye. These active sunspots have been the source of strong flares including at least one X class flare. The resulting CME’s have sparked displays of aurora over the last few days.

A parade of large sunspots crossing the disk of the Sun on 11 Aug 2024
A parade of large sunspots crossing the disk of the Sun on 11 Aug 2024

The equipment is capable of yet better images, but I would need better conditions than the poor seeing we usually get in Waikoloa. Perhaps load up the ‘scope and travel to higher ground.

A Busy Sun

Solar maximum is upon us and the Sun is a very busy place these days. To the delight of those of use who watch, a parade of large sunspots can be observed crossing the Earth facing side. These magnetic tangles have also been releasing flare after flare, sometimes causing strong araoras here on Earth.

It is cetianly worth the effort to drag the ‘scope into the driveway and take a look. Unfortunately the seeing at the house was poor this morning so the resulting photos are not as sharp as I would like…

A parade of large sunspots crossing the disk of the Sun on 10 Aug 2024.
A parade of large sunspots crossing the disk of the Sun on 10 Aug 2024.

A Misty Night at Kaʻohe

While I have gotten plenty of telescope time lately, it has usually been morning sessions with the old Astrola in my driveway. This is a low effort and thoroughly enjoyable practice that I engage in about half a dozen times each month. Such sessions do mean that my 18″ telescope languishes for far too long in the garage.

Ben Harmon checking the sky in anticipation of a good evening of observing at Kaʻohe
Ben Harmon checking the sky in anticipation of a good evening of observing at Kaʻohe

I really need to change that.

Thus when my fellow staff at Symbrosia start asking for another star party it made a good excuse to get the big ‘scope out of the garage and under a dark sky.

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A Leap Day

Today is February 29th, that odd date that only occurs every four years.

The reason for a leap day inserted into the calendar, the existence of February 29th, is ultimately astronomical. Perhaps a little explanation is in order…

We originally defined days as the time it takes the Earth to rotate. While we define years as the time it takes the Earth to orbit once around the Sun. The problem is that these values do not divide evenly into one another.

Mauna Kea Sunrise
Sunrise seen from the summit of Mauna Kea
The Earth takes about 365.24219 days to obit the Sun, when measured by the Sun’s position in the sky, what is called a tropical year. There are different ways to measure a year, but if one is concerned with keeping the seasons in sync with your calendar, then you are interested in tropical years.

It is that bunch of decimals, the 0.24219 etc., that is the problem, every four years the count drifts out of sync by roughly one day. The insertion of an extra day every four years helps bring the calendar back into synchronization with the orbit of the Earth and with the seasons.

Even leap years do not quite fix the problem as 0.24219 is close, but not quite 0.25 or one quarter of a day. Thus additional corrections are needed… Enter leap centuries.

Our current calendar was instituted by Pope Gregory XIII in 1582, setting up a standard set of corrections for the fractional difference between the length of a year and the length of a day. Scholars knew that errors had been accumulating in the calendar for centuries, resulting in a drift of several days. Religious authorities were concerned that this drift had displaced important celebration in the church calendar, in particular the celebration of Easter. After much argument it was decided to reform the calendar. The current solution was devised by a number of astronomers, including Aloysius Lilius, the primary author of the new system.

The Gregorian Calendar uses an extra day in February every four years, unless the year is divisible by 100, then there is no leap leap day that year. However, if the year is divisible by 400, then it is a leap year. While this may sound odd, it does create a correction much closer to the ideal value of 365.24219 days per year.

I am a geek, so let us put that into code…

Even this is not perfectly precise. The correction is close but will drift given enough time. The length of a tropical year also changes slowly over time. We will eventually have to add another correction to keep the calendar and the seasons in sync. But not for a few millennia, good enough, for now.

As 2024 is divisible by four and not divisible by 100, there will be a leap day added to the end of this February… Today.

A Changing Sky

Gazing up at the sky while reading the old texts one would not be amiss in believing that the stars never change. Indeed there are many who insist vehemently that the stars are eternal and unchanging. But the stars do indeed change, often quite visibly, sometimes within the span of a human lifetime.

VLT/SPHERE image of the star L2 Puppis and its surroundings
A VLT/SPHERE image of L2 Puppis showing the enveloping cloud of gas and dust representing an early stage planetary nebula. Image credit ESO/P. Kervella used under Creative Commons 4.0

One such star is L2 Puppis.

A bright star, one of the few naked eye variable stars that could be seen to fade and reappear without the aid of a telescope much like the far more famous stars Mira and Algol. On star charts the star is found prominently drawn at magnitude 4, buf if you attempt to locate it today you will not find it without the use of a telescope.

I first encountered this star quite recently while starhopping through southern Puppis with an 8″ telescope from the driveway. The chart showed two bright stars close together, L1 Pup and L2 Pup, while the view in the finder ‘scope showed only one bright star.

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