Category Archives: Sun

Supermoons

During the last few days the news are talking about the “Supermoon” happening on Monday 14th November. The reports (some examples here, here and here) say that “it will be the brightest Full Moon in years“. Even we at the Australian Astronomical Observatory have been asked about this “very rare phenomenon“. But how much is true about all of this?

Let’s take a look. First of all we should have clear that the Moon, as any other small body moving around a larger body, has an elliptical orbit.

Diagram explaining the movement of the Moon around the Earth. Not in scale. Credit: Ángel R. López-Sánchez. Moon image: Paco Bellido.

Diagram explaining the movement of the Moon around the Earth following an elliptical orbit and defining the perigee and the apogee. Not in scale. Credit: Ángel R. López-Sánchez. Moon image: Paco Bellido.

Planets also move around the Sun following elliptical orbits, as it was discovered by the great astronomer (and the first real astrophysicist in History, although he also had to work as an astrologer to get a salary) Johannes Kepler at the beginning of the 17th century.

This means that sometimes the Moon is closer to the Earth and sometimes it is farther from the Earth, just depending on where it is located within its orbit. The point on the Moon’s orbit closest to Earth is called the perigee (at an average distance of 362 600 km) and the point farthest away is the apogee (at an average distance of 405 400 km). On average the Moon-Earth distance is about 382 900 kilometers.

Therefore, just because of its distance, the apparent size of the Moon is a bit larger than usual when it is at the perigee, while it seems a bit smaller than usual when the Moon is at the apogee. An image can explain this much better than words:

Comparison of the apparent size of the Moon when it is located at the perigee (left) and when it is at the apogee (right). Credit: Paco Bellido.

Comparison of the apparent size of the Moon when it is located at the perigee (left) and when it is at the apogee (right). Credit: Paco Bellido.

These photos were taken by the Spanish astrophotographer and friend Paco Bellido in 2014 and 2015 from Córdoba (Spain), my natal city, and clearly show the different apparent size that the Moon has at the perigee (left) when compared to where it is at the apogee (right).

What does happen when the full moon coincides with the perigee? Well, that is a supermoon! The next time this will occur is next Tuesday, 15th November, 12:52am Sydney time. In that moment the Moon will be ~13% larger and ~30% brighter than a full moon happening in the apogee (a “micromoon“). From Sydney (and Australia) the best moment to see it will be on the evening of Monday 14th November, and actually many people are planning to enjoy watching the “supermoon” appearing over the Pacific Ocean at the dusk from Sydney’s famous beaches and clifts.

Regarding this, it is important to say that our brain tricks us when observing the Moon or the Sun close to the horizon: they do appear to be larger than they do higher up in the sky. This is called the Moon illusion, some studies suggest that the perception is that the Moon is almost 3 times larger near the horizon that when located near the zenith.

Supermoon over Espejo's Castle (Córdoba, Spain) on 20th March 2011. This photo, taken by Paco Bellido, has been widely used in many places since then. Now people still try to get it too with their cameras... More info (in Spanish) in "El beso en la luna". Credit: Paco Bellido.

Supermoon over Espejo’s Castle (Córdoba, Spain) on 20th March 2011. This photo, taken by Paco Bellido, has been widely used in many places since then. Now people still try to reproduce this photo with their cameras when full moon… More info (in Spanish) in Paco’s blog “El beso en la luna“. Credit: Paco Bellido.

However, I must insist that the term “supermoon” does not come from Astronomy but from the pseudoscience of astrology. Perhaps that is one of the reasons why many people are talking about this. The term “supermoon” was coined by the US astrologer Richard Nolle in 1979, who defined it as ‘a New or a Full Moon that occurs when the Moon is at or near (within 90% of) its closest approach to Earth in its orbit’.

Nolle, who associated supermoons to catastrophes without any scientific evidence that this was true, didn’t know that we astronomers already had a scientific term to describe this alignment: the perigee-syzygy of the Earth-Moon-Sun system. The word “syzygy” means a perfect alignment between three bodies, that are in a perfect straight line. The most famous examples of syzygies are the lunar and solar eclipses, when the alignment of the Sun, Earth and Moon happens on the lunar nodes (the two points where the plane of the orbit of the Moon around the Earth and the plane of orbit of the Earth around the Sun intercept).

As other “expressions”, such as “blood moon” (a lunar eclipse) or “blue moon” (the second full moon within the same calendar month), the term “supermoon” has become very popular lately, perhaps also because all the action in social media. But these definitions are not official astronomical terms. Indeed, a “blue moon” does not have a proper astronomical definition, and may happen or not depending on the time zone the observer is located.

In any case all the excitement about the supermoon happening on Tuesday 15th (for us in Sydney, but for the majority of the world on Monday 14th) it that the exact moment of the full moon (12:52 am Sydney time) is really close to the perigee, happening at a distance of only 356 536 km from us. The supermoon was not that close since 26th January 1948, when it was at 356 460 km, and it will not be that close till 26 November 2034, when it happens at 356 472 km.

Check the numbers, please. 356 532 km, 356 460 km, 356 472 km… they all just differ in tens of kilometers! That is only a difference of a 0.02% ! Even considering the distances happening on other supermoons (I forgot to say we typically have 2-3 supermoons per year, last 17th October and next 13 Dec will be also supermoons), the differences are just within around 500 km, what is translated into a difference of only 0.14%.

Illustration: Supermoons: can you see what is the largest? Eight supermoons between 2015 and 2018, images have been scaled to the apparent size of the Moon considering its distance from Sydney when the full moon is happening. The dates are times indicated are the moment of the Full Moon. The sizes and distances are computed assuming the observer is located in Sydney, Australia. This is an illustration, not real photos taken from Sydney (I can't travel to the future!). The original Moon image is the photo of the "micromoon" that Spanish astrophotographer Paco Pellido took on 5 March 2015 from Córdoba, Spain, which is the image I use in this post. An image without labels can be found here. The high resolution image is available here. Credit: Ángel R. López-Sánchez, Moon Photo Credit: Paco Bellido.

Illustration: Supermoons: can you see what is the largest? Eight supermoons between 2015 and 2018, images have been scaled to the apparent size of the Moon considering its distance from Sydney when the full moon is happening. The dates are times indicated are the moment of the Full Moon. The sizes and distances are computed assuming the observer is located in Sydney, Australia.
This is an illustration, not real photos taken from Sydney (I can’t travel to the future!). The original Moon image is the photo of the “micromoon” that Spanish astrophotographer Paco Pellido took on 5 March 2015 from Córdoba, Spain, which is the image I use in this post. An image without labels can be found here. The high resolution image is available here. Credit: Ángel R. López-Sánchez, Moon Photo Credit: Paco Bellido.

Let me say it again: the difference of the distance between the Earth and the Moon during a “supermoon”, with these happening typically 2-3 times per year (for full moon, 4-5 times per year in total including new moon), is only the 0.14%. Do you think you’ll be able to notice this with your naked eye?

However, giving numbers (talking quantitatively) the media can say “it is a rare event, the closest supermoon in almost 70 years“. But in practice you’ll not notice a thing. It will be a supermoon essentially similar to all of those we have every year.

Distance from the observer to the Moon depending on when rising or setting (top) or when it is near the zenith (bottom). Credit: Ángel R. López-Sánchez. Moon image: Paco Bellido.

Distance from the observer to the Moon depending on when rising or setting (top) or when it is near the zenith (bottom). Credit: Ángel R. López-Sánchez. Moon image: Paco Bellido.

There is more. Besides the lunar illusion, the moon is actually a bit further away from us when it is rising or setting than when it is near the zenith, as the image above clearly shows. The difference on the distance between the observer and the Moon may vary between few thousand an twelve thousand kilometers. This is called “diurnal effect” as it is, indeed,  larger than the difference of few hundreds of kilometers found for supermoons. In both cases, I insist, the differences on the apparent size of the Moon can’t be noted with the naked eye.

Here again it is important to have a bit of critical thought about what all of this means. In any case this “supermoon” is a great excuse to forget about our domestic problems, look at the sky and be amazed by all the beautiful things that are hiding among the stars.

More info:

PS: Ah, yes, a curiosity:  it is me who will be observing at the Anglo-Australian Telescope (AAT) the night of Monday 14th till Tuesday 15th… That is, quantitatively talking this will be the worst night since the AAT was built to be observing there…

Update 17 November:

I’ve included the illustration comparing the size of the Moon for 8 supermoons, as seen from Sydney. This started as a game in social media on Monday. I also prepared this illustration showing the sizes of the 12 full moons in 2016, as seen from Sydney. Do you identify the micromoon and the 3 supermoons?

Illustration: Full Moons in 2016 as seen from Sydney. All the full moons in 2016, scaled in size following the Moon's apparent size as seen from Sydney. The micromoon corresponds to 22nd Apr (top right) and the thre supermoons are 16 Oct, 14 Nov (15 Nov Sydney time) and 14 Dec. This is an illustration, not real photos taken from Sydney (I can't travel to the future!). The original Moon image is the photo of the "micromoon" that Spanish astrophotographer Paco Pellido took on 5 March 2015 from Córdoba, Spain, which is the image I use in this post. The image without labels is here. A high resolution image is available in my Flickr. Credit: Ángel R. López-Sánchez. Moon photo credit: Paco Bellido.

Illustration: Full Moons in 2016 as seen from Sydney. All the full moons in 2016, scaled in size following the Moon’s apparent size as seen from Sydney. The micromoon corresponds to 22nd Apr (top right) and the thre supermoons are 16 Oct, 14 Nov (15 Nov Sydney time) and 14 Dec. This is an illustration, not real photos taken from Sydney (I can’t travel to the future!). The original Moon image is the photo of the “micromoon” that Spanish astrophotographer Paco Pellido took on 5 March 2015 from Córdoba, Spain, which is the image I use in this post. The image without labels is here. A high resolution image is available in my Flickr. Credit: Ángel R. López-Sánchez. Moon photo credit: Paco Bellido.

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Seasons: Astronomy vs. Australia

Something that really shocked me when I started to live in Australia ~7 years ago was to hear everywhere that seasons start at the beginning of the corresponding month. That is, as today is Sep 1st, everyone in the radio / advertisements / news is welcoming Spring. And this, being an astronomer, believe me, hurts. Why? Because astronomically we are still in winter. Seasons are defined by Astronomy in a very accurate and precise way. This year Spring starts on September 23rd, 11:29 AEST (02:29 Universal Time). That is when that the Autumn Equinox happens, and the real moment Spring starts in the Southern Hemisphere (and Autumn/Fall starts in the Northern Hemisphere).

The seasons are caused by the combination of three astronomical factors: the Earth’s is a (almost perfect) sphere, the Earth’s orbit around the Sun, and the Earth’s axial tilt. As a consequence of these the Earth’s atmosphere is unequally heated by the Sun around the year at a given position. Therefore, the seasons are marked by the movement of the Earth around the Sun and, hence, which way the Earth is tilted with respect to the Sun. When the South hemisphere is tilted towards the Sun, the Sun’s rays strike the Earth at a steeper angle compared to a similar latitude in the North hemisphere. As a result, the radiation is distributed over an area which is less in the South hemisphere than in the North hemisphere. This means that there is more radiation per area to be absorbed in the South hemisphere, and therefore it is winter in the North hemisphere and summer in the South hemisphere.

Illumination of Earth by Sun at the southern solstice. Credit: Wikipedia


By astronomical definition, the precise timing of the seasons is determined by the exact times of transit of the Sun over the tropics of Cancer and Capricorn for the solstices and the times of the Sun’s transit over the Equator for the equinoxes, as specified in this figure:

Movement of the Earth around the Sun following an orbital ellipse (with eccentricity exaggerated for effect) and seasons. Equinoxes (20 or 21st March and 22nd or 23rd September) happen when the tilt of Earth’s axis neither inclines away from nor towards the Sun (green dotted line), and hence two points a the same latitude but a different hemispheres receive the same amount of energy from the Sun. In an equinox, the Sun is found at the zenith at the midday at the Equator. A solstice (20th or 21st June and 21st or 22nd December) happens when the tilt of the Earth’s axis has maximum effect (23.44º, red dotted line). At the June solstice the Sun is found at the zenith at the midday (just over our head!) at latitude 23.44º North, defining the Tropic of Cancer. Similarly at the December solstice this happens at 23.44º South, known as the Tropic of Capricorn. The periapsis (perihelion) and the apoapsis (aphelion) mark the nearest and the farthest points from the Sun, respectively (blue dotted line). Credit: Wikipedia


Therefore, in the South hemisphere, Spring starts with the Autumn Equinox, Summer with the Winter Solstice, Autumn with the Spring Equinox and Winter with the Summer Solstice. Of course, the names were given as correct for the North hemisphere.

Well, at least all of this is what Astronomy says. However, Governments and societies quite often decide to use their own definitions. Just checking this webpage of the Australian Bureau of Meteorology:

In Australia, the seasons are defined by grouping the calendar months in the following way:

1. Spring – the three transition months September, October and November.
2. Summer – the three hottest months December, January and February.
3. Autumn – the transition months March, April and May.
4. Winter – the three coldest months June, July and August.

These definitions reflect the lag in heating and cooling as the sun appears to move southward and northward across the equator. They are also useful for compiling and presenting climate-based statistics on time scales such as months and seasons.

Following these assumptions, Australia indeed enters in Spring today, which is funny because the majority of the countries (if not all) of the North hemisphere are still in Summer. In any case, for me it is Winter, and it will be winter till next on September 23rd, 11:29 AEST, when Spring, according to Astronomy, really starts.

First “AAO Guerrilla Astronomy” event: partial solar eclipse on 29 April 2014 over Sydney Harbour

Last Tuesday 29th April the Earth, the Moon, and the Sun aligned to produce one of the most spectacular astronomical phenomena we can see: a solar eclipse. The 29th April solar eclipse was actually not a total eclipse (i.e., the disc of the Moon didn’t cover all the disc of the Sun) but an Annular eclipse. The annular phase could be only visible in Antarctica, but a partial solar eclipse was seen throughout Australia in the late afternoon. More information about this solar eclipse can be found in the NASA Eclipse Website managed by the astrophysicist Fred Espenak.

The Sun would be eclipsed by the Moon during the sunset, it was then a perfect opportunity to get some nice photos of the eclipsed Sun with some famous buildings such the Sydney Opera House or Sydney Harbour Bridge. With this excuse, but also with the idea of showing the wonders of Nature to the public, a group of astrophysicists working at Australian Astronomical Observatory (AAO) decided use this solar eclipse to organize our first “Guerrilla Astronomy” event (*). The aim of these activities is to set up amateur telescopes in a public area (a park or a shopping center) and explain to the public who is around what Astronomy is, what astronomers do, and what the “Australian Astronomical Observatory” is. More of these events are coming in the future, but this was our first “test” to see how we can organize and manage the activity.


Participants to the first AAO “Guerrilla Astronomy” Event. From right to left, Stuart Ryder (AAO/AusGO), Kyler Kuehn (AAO), Paola Oliva-Altamiro (Swinburne/AAO) and Ángel R. López-Sánchez (AAO/MQ). The laptop shows the only good image we could get of the eclipse using my telescope. Mrs Macquarie Chair, Sydney Botanic Gardens / Domain, 29 Apr 2014.
Photo Credit: Stuart Ryder (AAO/AusGO).

Given the time and position of the Sun during the eclipse, we decided that a really nice spot to prepare our telescopes would be Mrs Macquarie Chair point, in the Domain, Sydney Botanic Gardens. From there a very dramatic view of the Sydney Opera House and the Sydney Harbour Bridge is seen. We first requested permission to do this to the authorities of the Domain, who were really nice and even allowed us to park by free. Actually, they also came along to see the eclipse and they liked our idea of organizing more “Guerrilla Astronomy” events there in the nearby future.


All set up for eclipse: two telescopes (Stuart’s at the right, mine at the left), the AAO banner, my laptop and camera to take photos through the telescope, the eclipse glasses and extra information about the eclipse to give to the visitors. Mrs Macquarie Chair, Sydney Botanic Gardens / Domain, 29 Apr 2014.
Photo Credit: Ángel R. López-Sánchez (AAO/MQ).

It was four of us, Stuart Ryder (AAO/AusGO), Kyler Kuehn (AAO), Paola Oliva-Altamiro (Swinburne/AAO) and myself, who participated in this first “Guerrilla Astronomy” event. Just to have everything ready on time, we were setting up telescopes, AAO banner and laptop around an hour before the beginning of the eclipse. The weather seemed very clear in the morning, but in the afternoon, as we feared, some clouds started to arrive from the west. We already knew this would be a killer… but we had to try!


Kyler and visitor using the solar glasses. First AAO “Guerrilla Astronomy” Event: partial solar eclipse on 29 April 2014 over Sydney Harbour. Mrs Macquarie Chair, Sydney Botanic Gardens / Domain.
Photo Credit: Paola Oliva-Altamiro (Swinburne/AAO).


Little girl using the eclipse glasses. First AAO “Guerrilla Astronomy” Event: partial solar eclipse on 29 April 2014 over Sydney Harbour. Mrs Macquarie Chair, Sydney Botanic Gardens / Domain.
Photo Credit: Paola Oliva-Altamiro (Swinburne/AAO).


Visitors, but clouds please go away! First AAO “Guerrilla Astronomy” Event: partial solar eclipse on 29 April 2014 over Sydney Harbour. Mrs Macquarie Chair, Sydney Botanic Gardens / Domain.
Photo Credit: Paola Oliva-Altamiro (Swinburne/AAO).

We actually were a bit lucky at the beginning, and hence we could see the Sun within thin clouds and follow the eclipse for 10 minutes. I even could take a nice image:


Partial Solar Eclipse from Sydney on 29 Apr 2014. Telescope Skywatcher Black Diamond D = 80 mm, f = 600 mm + CANON EOS 600D at primary focus + Solar filter. Just 1 frame at ISO 400, 1/8 s, colour processing using Photoshop. 29 April 2014 @ 16:20 AEST ( 06:20 UT ). First AAO “Guerrilla Astronomy” Event: partial solar eclipse on 29 April 2014 over Sydney Harbour. Mrs Macquarie Chair, Sydney Botanic Gardens / Domain.
Photo Credit: Ángel R. López-Sánchez (AAO/MQ).

After that, thick clouds arrived and this happened:

5-seconds timelapse video obtained combining 25 images taken with Telescope Skywatcher Black Diamond D = 80 mm, f = 600 mm + CANON EOS 600D at primary focus + Solar filter, at ISO 400, 1/8 s, showing how the clouds completly cover the eclipsed sun. 29 April 2014 @ 16:20 AEST ( 06:20 UT ). The direct link to the YouTube video is here.
Credit: Ángel R. López-Sánchez (AAO/MQ).

Once the Sun was completely covered by thick clouds we just waited and hoped for a little gap, but unfortunately this never happened and we didn’t see the Sun again that day.


Stuart and his telescope, Kyler and visitors, all hoping the clouds go away. First AAO “Guerrilla Astronomy” Event: partial solar eclipse on 29 April 2014 over Sydney Harbour. Mrs Macquarie Chair, Sydney Botanic Gardens / Domain.
Photo Credit: Paola Oliva-Altamiro (Swinburne/AAO).


The eclipsed sun is setting behind those think clouds. First AAO “Guerrilla Astronomy” Event: partial solar eclipse on 29 April 2014 over Sydney Harbour. Mrs Macquarie Chair, Sydney Botanic Gardens / Domain.
Photo Credit: Ángel R. López-Sánchez (AAO/MQ).

Well, it would have been really nice to see the eclipsed sun setting over the Sydney Harbour Bridge and sinking later close to the Sydney Opera House, I’m sure the images and time-lapse video would have been quite spectacular, but the best I got was this image:


An eclipsed sun should be setting around there… Imagen taken using a Telescope Skywatcher Black Diamond D = 80 mm, f = 600 mm + CANON EOS 600D at primary focus. First AAO “Guerrilla Astronomy” Event: partial solar eclipse on 29 April 2014 over Sydney Harbour. Mrs Macquarie Chair, Sydney Botanic Gardens / Domain.
Photo Credit: Ángel R. López-Sánchez (AAO/MQ).

In any case, all four AAO participants were very happy about how the event was and, as I said, we are expecting to repeat these “Guerrilla Astronomy” activities in the nearby future.

Next solar eclipse to touch Australia will be on 9 March 2016, but it will also be a partial eclipse only visible on the northern and western parts of the continent. The next total eclipse to be seen from Australia will happen on 20 April 2023 and it will just touch the coast of Western Australia. We have to wait until 22 July 2028 to see a total solar eclipse in Sydney. Actually, Sydney is almost exactly in the center of the totality.

More photos of this event can be found in this Flickr Album.

(*) Note that the word “Guerrilla” comes from Spanish, however the name didn’t come from me but from an idea my colleague Amanda Bauer (AAO Outreach Officer) had some months ago. As a native Spanish speaker I have to confess it is really hard to hear the pronunciation of “Guerrilla” following English phonemes as “Guerrilla Astronomy” sounds almost identical toGorilla Astronomy“. I would encourage to try to pronounce “Guerrilla” as it is said in Spanish (geˈri.ʝa) to be released of this confusion, but of course that is only my modest suggestion than can be completely ignored…

Partial solar eclipse from Sydney

Today, 10th May 2013, the combined movements of the Sun, Earth and Moon gave us a very nice Annular Solar Eclipse. Following a similar path to the Total Solar Eclipse we enjoyed last November, the shadow of the Moon over the Earth moved from North Australia to the Pacific. However, today the Moon was close to its maximum distance to the Earth (planets and satellites move following elliptical orbits) and hence its apparent size on the sky was not big enough to completely cover the disc of the Sun. This is indeed the reason the eclipse was an annular solar eclipse.

In this occasion I couldn’t travel to North Australia to enjoy the annular eclipse (actually, I have seen 2 of these in the past, the most recent one was on 3rd October 2005 from Madrid), and even last night I didn’t expect to do anything special about this today. But this morning, while watching it from my backyard using my solar glasses, I decided just to take some few shots using not the telescope but only the tele lens. This is the result:

Partial Solar Eclipse from Sydney. Data obtained using a CANON EOS 600D, a 250mm Tele Lens and a Solar filter (which I hold by hand). I stacked 12 individual frames obtained at ISO 100, f10, 1/80 s using the Lynkeos software. The final processing was achieved using Photoshop. 10 May 2013 @ 09: 10 AEST ( 00:10 UT ), Sydney, Australia.
Credit: Ángel R. López-Sánchez (Australian Astronomical Observatory / Macquarie University, Agrupación Astronómica de Córdoba / Red Andaluza de Astronomía)

I hope you like it.

Timelapse of the Total Solar Eclipse

Last week I shared some of the images I obtained during the Total Solar Eclipse on 13 / 14 November 2012. It was observed from the Mulligan Highway, 44 km south of Lakeland, Queensland Australia. After spending a weekend playing with the raw frames, I ended up with this timelapse video, which shows all the sequence of the eclipse.

Timelapse video of the Total Solar Eclipse on 13 / 14 Nov 2012. The direct link to YouTube is here. Credit: Ángel R. López-Sánchez (Australian Astronomical Observatory / Macquarie University, Agrupación Astronómica de Córdoba / Red Andaluza de Astronomía).

The video combines 1203 individual frames obtained while the eclipse was happening. As before, I used my refractor Skywatcher telescope, 80 mm aperture and 600 mm focal, and my digital camera CANON EOS 600D at primary focus. For all partial phases but the totality I used a solar filter which blocks the 99.9997% of the incident light. The approximate field of view of the video is 2ºx1º. I usually took a frame each 6 seconds, but sometimes I triggered many consecutive images to improve the quality of the final photo of that moment. The music is the theme “WorldBuilder” written by Fran Solo and included in Epic Soul Factory Xpansion Edition.

Total Solar Eclipse 13 / 14 Nov 2012

After many years waiting for it, I have finally observed (and enjoyed!) my very first Total Solar Eclipse. It was on 14 November 2012 (still 13 November following time in UT) and I was 45 km south of Lakeland, Queensland Australia (I had to drive during the night trying to escape from the clouds in the coast near Port Douglas). Here you have some of the images I have obtained of this rare phenomenon.

My sequence of the Total Solar Eclipse on 13 / 14 November 2012, 50 km south from Lakeland, Queensland, Australia. I used a Skywatcher D 80mm, F 600mm, primary focus using CANON EOS 600D. All times given in UT and correspond to 13 Nov 2012. Credit: Ángel R. López-Sánchez (Australian Astronomical Observatory / Macquarie University, Agrupación Astronómica de Córdoba / Red Andaluza de Astronomía).

Some more pictures:

The sun rises, but the eclipse did already start. Credit: Ángel R. López-Sánchez (Australian Astronomical Observatory / Macquarie University, Agrupación Astronómica de Córdoba / Red Andaluza de Astronomía).

Image of the totality showing the brightest areas of the solar corona and some solar prominences close to the lunar limb (in red). Credit: Ángel R. López-Sánchez (Australian Astronomical Observatory / Macquarie University, Agrupación Astronómica de Córdoba / Red Andaluza de Astronomía).

Image of the totality showing the diffuse solar corona, but the brightest areas are overexposed. Credit: Ángel R. López-Sánchez (Australian Astronomical Observatory / Macquarie University, Agrupación Astronómica de Córdoba / Red Andaluza de Astronomía).

Diamond ring, the first light of the Sun coming after the totality. Credit: Ángel R. López-Sánchez (Australian Astronomical Observatory / Macquarie University, Agrupación Astronómica de Córdoba / Red Andaluza de Astronomía).

HDR (High Dynamic Range) image combining 20 individual frames with different exposition times. Credit: Ángel R. López-Sánchez (Australian Astronomical Observatory / Macquarie University, Agrupación Astronómica de Córdoba / Red Andaluza de Astronomía).

What are Wolf-Rayet stars?

Wolf-Rayet (WR) stars are the evolved descents of the most massive, extremely hot (temperatures up to 200,000 K) and very luminous (105  to 106 solar luminosities, L) O stars, with masses 25 – 30 solar masses (M) for solar metallicity. WR stars possess very strong stellar winds, which reach velocities up to 3,000 km/s. These winds are observed in the broad emission line profiles (sometimes, even P-Cygni profiles) of WR spectra in the optical and UV ranges. These strong winds are also attributed to atmospheres in expansion. Actually, these winds are so strong that they are peeling the star and converting it in a nude nucleus without envelope. Indeed, WR stars have ejected their unprocessed outer Hydrogen-rich layers. WR stars typically lose 10−5 M a year; in comparison the Sun only loses  10−14  M⊙  per year.

Hα image of the Population I Wolf-Rayet star WR 124 (WN8) showing a young circunstelar envelope that is ejected at velocities highest than 300 km/s. The chaotic and filamentary structure created forms the M 1-67 nebula. The star is located at about 4.6 kpc from the Sun. At the left, image obtained by the author using the IAC-80 telescope, combining filters Hα (red) Hα continuum (green) and [O III] (blue). The right Hα image was obtained by the Hubble Space Telescope WFPC2 (Grosdidier et al. 1998). Note that the large arcs of nebulosity extend around the central star yet with not overall global shell structure. Furthermore, numerous bright knots of emission occur in the inner part of the nebula, often surrounded by what appear to be their own local wind diffuse bubbles. The dashed square in the IAC-80 image indicates the size of the HST image.

This is Figure 2.1 in my PhD Thesis.

WR stars were discovered by French astronomers Charles Wolf and Georges Rayet in 1867. They found that three bright galactic stars located at Cygnus region have, rather than absorptions lines, broad strong emission bands superposed to the typical continuum of hot stars. In 1930 C.S Beals correctly identified these features as emission lines produced by high ionized elements as helium, carbon, nitrogen and oxygen.  The intriguing spectral appearance of WR stars is due both their strong stellar winds and highly evolved surface chemical abundance. In 1938, WR stars were subdivided into WN (nitrogen-rich) and WC (carbon-rich) depending on whether the spectrum was dominated by lines of nitrogen or carbon-oxygen , respectively. Not until the 1980s did it became clear that WR stars represent an evolutionary phase in the lives of massive stars during which they undergo heavy mass loss. 

The mass-loss occurs via a continuous stellar wind which accelerated from low velocities near the surface of the star to velocities that exceed the surface escape speed. Their spectra, originated over a range of radii with the optical continuum forming close the stellar core and the emission lines in the more external areas (even beyond 10 stellar radii), indicate that the WR stars are embedded in luminous and turbulent shells of ejecta owing outwards at speeds comparable to the expansion velocities of novae although, in the case of WR stars, the expanding shell is being constantly fed with material from the main body of the star.

WR stars are extremely rare, reflecting their short lifespan. Indeed, they live for only some few hundred of thousands years, and hence only few WR stars are known: about 500 in our Milky Way and 100 in the Large Magellanic Cloud. Indeed, because of their peculiarities (brightness and broad emission lines), WR stars can be detected in distant galaxies. A galaxy showing features of WR stars in its spectrum is known as a Wolf-Rayet galaxy.

I compiled the main characteristics of WR stars in Chapter 2 of my PhD Thesis. A recent review about the properties of WR stars was presented by Crowther (2007).