## Blinking cepheid variable stars in Messier 13

Something that feels like a once in a lifetime event happened here in Bergen this spring. We had four consecutive days of clear skies, with very little wind and no moon!

Earlier on the blog I have expressed the desire to try and image Messier 13 on consecutive nights, to see if I could see the difference in magnitude in the variable stars in the cluster. Especially the variable star V1553 Her, which has a very convenient period of approximately 5 days. 4 days of observing would get me most of the period.

These 4 days were pretty late in spring (30th of March to 2nd or April), and observing had to be done after 10PM in a work week. Not ideal. Thankfully except from the usual tracking problems, there were few problems with the equipment. My own patience failed while doing focusing though, which resulted in two nights with good images, and two with not so great quality.

This was a bit problematic since I wanted to compare images over time, and since I had 40 good images from one night, and 10 from another there was a big difference in the brightness of the stars once I was done stacking images from each night. To account for this I did some brightness matching in post processing matching on the non variable stars of each frame.

The resulting 4 frame time-lapse looks like this.

I was pretty happy with this, as it shows the variability of not only one, but at least one more, and maybe even a third variable star I missed at first (Can you find it? This paper has charts to help). The change in brightness especially for V1553 Her was also much clearer then I expected. Great success!

The process to find the distance to the star from the magnitude and variability data is neat. Since V1553 Her is a Type II Cepheid (how this was determined is not clear to me, please add a comment if you know) which period is approximately 5 days the star has an absolute magnitude of approximately -1.5 according to this chart. The formula below (from here), should then give the distance $$d$$.

$$M_v = m – 2.5log((d/10)^2)$$

Looking at the images and the reference stars, a rough estimate would be that V1553 Her varies between 12 and 13 in apparent magnitude. Plugging in -1.5 for $$M_v$$ and 12.5 for $$m$$ gives the distance of 6310 parsecs, which is 500 parsecs off.

While I have not followed a very thorough process here, it is anyway nice to verify that my data seems to fit to reality. To get better data for deep space objects, I am evaluating to get a solid equatorial mount with either a small refractor or an 8 inch newtonian for imaging, and keep my dobsonian for visual and planetary. Once I do I hope a can revisit this project and get a time-lapse with much higher quality. Maybe do one which would also capture the really fast variables with down to 0.2 days period.

## Observation log 18. of April 2021.

Conditions were not great, moonlight was a pain, and some red light leaked from some camera status light into all of my M3 images.

On the positive side I finally got to see M3 and M5 and also M57 (Ring nebula), and I got decent images (for me anyway) of M13 and the leo triplet.

## M57 (Ring Nebula)

I have long wanted to try to observe a planetary nebula, but I think I had the size all wrong. I was looking for something larger.

In my finder scope M57 looked a lot like a star, and if it was not so easy to locate due to being in between two bright stars I would have probably scanned past it. Once I found it, the ring structure was clearly visible at medium magnification. Really neat!

I think that having calibrated my expectations now, finding other planetary nebulae will be easier.

### M13 (Hercules globular cluster)

Earlier this year I ordered a 2x TV Powermate mainly for planetary observation. To test it I tried using it together with my 2″ Aero on the globular clusters and M57. It worked way beyond my expectations.

I also took 15 seconds exposure of M13 below (the most my tracking was good for). The two red spots I marked are two variable stars, which I found comparing the chart here to my image.

These stars have a periodic change in brightness. The period of their change in brightness determine their absolute brightness. Then the relative brightness (the brightness observed from Earth) and the absolute brightness can be used to find the distance to the star.

It would be really nice to try and observe M13 with the same magnification over time, and see if I can catch the blinking.

### Leo triplet

I observed these galaxies visually some time ago, but this time I got to take a series of pictures as well. Out of like around 20 exposures of 15 seconds, I got 6 images that were fine. After stacking this was the result. Noisy, but some structure at least.

I had a lot of fun this night!

## Less light pollution; better photos.

So I finally got to try to take astro photographies from somewhere with less light pollution. I still have trouble with movement from either my tracking lagging a bit, or me not having a remote control for the camera (so I cause movement in the telescope when I start the shot).

## M42 – Orion Nebula again

M42 is so easy to find observe and shoot. I also finally got to see some of the Flame Nebula, by doing a 30 seconds exposure into what looked like nothing at all. It sadly did not turn out very well because of movement in the camera, but I finally saw something! Anyway, here is my M42 shot. Getting better at this!

## M31 – Andromeda Galaxy

The moon would easily fit in this picture, but the Andromeda galaxy takes more space, so this is mostly the core and some of the arms. I think I could fit most of the galaxy if I had rotated the camera. I randomly also caught M32 (barely visible at the left edge) and M110 (in the lower right corner). Looking forward to try Andromeda with an even longer exposure, or many stacked images.

## Astro log – Through the smog and light pollution

A month back I finally got a new stepper motor and got the tracking for my telescope working. One dark night I took it to a pretty dark spot close to Bergen and did some observations.

## M42 – Orion Nebula

I have observed the Orion nebula under bad light pollution before, but this time I got to observe it with under better conditions. It was stunning. Very sharp.

The next day I took my first deep space photography ever from our apartment. The light pollution was really bad, and there was some smog as well. I also forgot that I could use a timed shot. So i think some blurring is due to the camera moving slightly after I started the exposure.

I think it turned out fine for a first:

## M1 – Crab Nebula

My original plan for the trip, was to observe M42 and the Andromeda Galaxy. I was also hoping to get to see the Flame Nebula since it was really dark. I sadly did not see any trace of the Flame Nebula so I started looking for some open clusters to look at in Taurus. While scanning for them I suddenly saw that the Crab Nebula was close, and I found it immediately. It is the first supernova remnant I have observed, and I think I saw some small amount of detail. Hoping to get a picture of it one of these days.

## M31 and M110 – Andromeda Galaxy and a friend

Andromeda is a not that interesting to visually observe since it is so hard to see anything beyond the core. I think I saw some more since it was really dark, but it was very faint. These two are prime targets for a photo some day, since that should bring out some more detail.