We are approaching the 2021 Vernal Equinox and are witnessing how fast the daylight hours are increasing for observers at 49 N. This phenomenon will affect the visibility of certain stars more than others. Sirius, the brightest star in the sky will be greatly affected by this day lengthening and will go quickly into its summer sleep around mid-May and stay out of sight for about three months for observers at 49N. On the other hand, the mighty red giant Antares is just “warming up” for its’ “opposition” with the Sun in late May – early June, when it will dominate the low southern skies.
The very bright star towards the upper left corner of the frame is Antares. Credit: ESO/B. Tafreshi (twanight.org), CC BY 4.0 https://creativecommons.org/licenses/by/4.0, via Wikimedia Commons
So, which of these two famous stars has better visibility for observers at mid-northern latitudes like 45 N or 49 N?
Many would say that Sirius, being almost 10 degrees higher in the sky for northern hemisphere observers is the absolute favourite. But hold on a sec, Antares is not throwing the towel in yet.
The chart below shows the hours of visibility for Sirius and Antares for each Friday in 2021. On the first sight the blue bars dominate the red ones, especially in the months when Sirius is visible for almost 10 hours each night.
Visibility hours for Sirius compared to Antares throughout 2021, courtesy od Sky Safari and Milan B.
But, on a closer look, the gap of invisibility for Sirius seems much wider than the one for Antares, revealing the fact that Sirius is invisible for much longer than Antares.
So, to answer the above question we need to refine the definition of “better” visibility. If we add up all visibility hours throughout the year, we can see that Sirius’ total hours dominate. This is confirmed by the average (for the year) line for Sirius, which is close to 5 hours per day compared to the average line for Antares, which is at around 4 hours per day. However, if we add up all days when each star is visible, then Antares becomes an unexpected winner. It is out of sight for observers at 49N only for about three weeks in late November – early December when the late autumn sun slides just above it, on its steady stroll along the ecliptic.
It is important to mention that for the reason of simplicity, the visibility hours in the above chart have been calculated when the star is above the horizon while the Sun is below the horizon. To compensate for the fact that stars are not visible immediately after rising or before setting, especially if the Sun is not far below the horizon, a one hour correction line was added to the chart. This line will “bite” a lot more into the visibility of Antares, as the mighty red giant spends more time in very low altitudes of just a few degrees above the horizon compared to Sirius.
Even if we subtract three weeks on each end of Antares’ “conjunction” with the Sun, which falls around Nov 30th, Antares will be the winner in this category.
It is also worth mentioning that the visibility in “wee” hours (after 1 AM) is being treated equally to the visibility at more friendly hours such as early evening. If we took just the observability at “normal” hours, when each star is not hugging the horizon, the outcome might be totally different.
Milan B, avid sky observer with both SkyWatcher and SkySafari.
The latest edition of our NOVA newsletter is available as a pdf file. An archive of older issues can be found on our Newsletter page. The contents include:
Strange New Worlds: Is Earth Special? (Paul Sykes Lecture, Thurs, Apr 8 @ 7:30pm, Dr. Phil Plait, The Bad Astronomer)
Life on Mars? by J. Karl Miller
President’s Message by Gordon Farrell
Astronomical Events in the Remainder of March by Robert Conrad
Update: Hoping for better weather tomorrow. SFU Trottier Observatory is going to go ahead with a stream tomorrow Tues Dec 22nd from 3:30pm to 5:30pm. https://youtu.be/vmoXUBUzjDk
RASC Global Star Party with Explore Scientific – 16:30 PST
RASC is partnering with Explore Scientific to bring you a star party of epic proportions! Explore Scientific will be livestreaming throughout the day on their channels (list and links available here). RASC members will be joining for the evening livestream, starting at 7:30pm EST. There will be presenters from across the country.
Celebrate the #GreatConjunction of #Jupiter and #Saturn. I will share my eyepiece with you as these two planets are 0.1° apart. Watch on @twitter @youtube or @Facebook as LivingSkyGuy. #astronomy #astronoMYtime #astrophotography
York University Allan I. Carswell Observatory: Jupiter and Saturn – The Great Conjunction of 2020 (ONLINE) – 13:00 PST
Announcing a Special Event at the Allan I. Carswell Observatory: Jupiter and Saturn – The Great Conjunction of 2020, Dec 21 from 4:00pm Toronto local time! A conjunction of Jupiter and Saturn only happens about once every 20 years (which is why it is called a great conjunction).
Great Conjunctions are pretty cool – Jupiter and Saturn line up and appear close together from our viewpoint. They occur somewhat rarely but regularly (about 20 years apart) due to the orbital periods of Jupiter (11.9 years) and Saturn (29.5 years). The next Great Conjunction, coming up in a few weeks on Dec 21st, 2020, is an extra-special one.
It is extra-special because Jupiter and Saturn will be extremely close together, just over 6 arc-minutes apart. You would have to go back almost 400 years to July 16th, 1623 to find them as close! To help visualize it, hold out your pinkie finder at arm’s length, that covers about 1°, so at conjunction, the two planets will be separated by a distance equal to about 1/10 the width of your pinkie – that is close enough that the two will appear as a single bright star to the naked eye. They will appear low to the horizon in the South-West around sunset on Dec 21st (sunset is at 4:15 pm PST).
Jupiter and Saturn will be low in the South-West, as viewed from Vancouver, BC on Dec 21st at 5:00 pm PST.
There’s no need to wait until Dec 21st as Jupiter and Saturn are already quite close together, starting off December about 2° apart. Both will easily fit within a 1° field of view (typical of common telescopes) from Dec 17th through to Dec 25th.
Saturn and Jupiter getting closer together as the Great Conjunction 2020 approaches on Dec 21. Image Credit: Sky At Night Magazine, Peter Lawrence.
Date
Separation (arc-minutes)
Dec 17
28
Dec 18
18
Dec 19
13
Dec 20
8
Dec 21
6
Dec 22
11
Dec 23
16
Dec 24
22
Dec 25
29
Jupiter and Saturn in a simulated 1° eyepiece field of view – click to open a larger version.
The low altitude and weather will be challenges for observing the conjunction from Vancouver. You may want to watch a live-streamed event from a remote location rather than betting on clear skies in December in Vancouver – Virtual Telescope, for example, is hosting a live-streamed event.
We are canvassing for one Executive position and one Council position. The position of National Representative is vacant and our Webmaster is seeking an assistant. If you wish to step onto council, please send an email to [email protected] to connect for follow-up.
The Agenda is as follows:
Meeting called to order
Acceptance of the Agenda
Reading of the 2019 AGM Minutes
President’s Report
Secretary’s Report
Treasurer’s Report
National Representative’s Report
Election of councilors in addition to the position of National Representative which is currently vacant for the remaining year of that position’s two-year term). If a member wishes to join council, they may step forward. Nominations for the aforementioned executive position can be taken from the floor as long as our bylaw requirements are met.
Supergiant stars, including both red and blue supergiants, are rare making up less than 1% of stars. Yellow supergiants are an even rarer but important subclass that includes prominent stars such as Polaris and δ-Cephei.
Our North Star,Polaris, is a Yellow Super Giant. Image credit: APOD Jan 11,2008, Steve Mandel & Research Collaboration: Adolf Witt (University of Toledo) et al.
Yellow Supergiants must meet two criteria: they have to be yellow with a spectral class of F or G, and they have to be bright with an absolute magnitude from about -5 to -8. It turns out that not many stars can satisfy both criteria for more than a short amount of time.
A Hertzsprung Russel Diagram shows how stars fall into different classes depending on their brightness and temperature. Image credit: http://collinspolarisstarslife.weebly.com/polaris-main-sequence.html
To understand them better, it is helpful to learn a bit about Hertzsprung-Russel Diagrams (H-R Diagrams) and stellar lifecycles. H-R diagrams help astronomers understand stellar evolution because stars fall into different positions and classes depending on where they are in their life cycle. Most stars, including our sun, spend most of their lifetime in the main sequence class where they produce energy by fusing hydrogen into helium. But as a star goes through its life stages, its luminosity and temperature change, hence its position on the H–R diagram also changes.
Our sun, for example, will spend about 10 billion years in the main sequence class and then expand and cool as it becomes a red giant. In doing so, its position on an H-R diagram will move up and to the right into the red giant class. The Sun will remain in the there for up to a billion years powered by the fusion of helium into carbon. After the helium is exhausted, the Sun will expel it’s outer layers as a planetary nebula then contract into a white dwarf. At this point, its position on the diagram moves into the white dwarf class where it remains for a long time. This lifecycle can be visualized as a path on an H-R diagram as shown below.
The Sun’s lifecycle as a Path on an H-R Diagram. Image credit: Carneiro, Robert L., Social Evolution & History. Volume 4, Number 1 / March 2005
Yellow supergiants, on the other hand, start off in the main sequence class and remain there for just a few million years. They live in the “Instability Strip” as a sort of pit stop on their way to becoming red giants. Stars in the Instability Strip oscillates between contracting/heating up and expanding/cooling down. This results in periodic variations in the star’s luminosity making them variable stars. In fact, most yellow supergiants are Cepheid Variables – an important class for determining stellar distances. The prototypical Cepheid variable, the star δ-Cephei in Cepheus, is a yellow supergiant.
In some cases depending on chemical composition, a red giant can heat up to become a yellow supergiant. This transition is called the “blue loop” as labelled in the H-R diagram below.
Evolution of a star 5X the mass of our Sun, showing a blue loop and other tracks across the yellow supergiant region Image Credit: Lithopsian, CC BY-SA 4.0 via Wikimedia Commons
Yellow supergiants only exist in the Instability Strip for a few thousand years. This short pit stop, coupled with the fact that 10+ solar-mass stars account for less than 1% of all stars explains the rarity of yellow supergiants. It is pretty cool that we can easily observe one with our naked eyes just by looking at Polaris, our prominent North Star.
Start observing Mars – Now, Today, or as soon as we get a clear night after the wildfire smoke clears out. The upcoming close approach and opposition of Mars, on Oct 6th and 13th, will likely provide the best views of Mars for the next 15 years as Mars will either be smaller or at a lower altitude during the next 6 oppositions. Plus, while the Southern Polar Cap is prominent now, it is melting and may disappear from view completely as we move later into October.
Stellarium chart showing the location of Mars at 10:00 pm PDT on Oct 13, 2020 from Vancouver, BC – in the south-east skies, about 30° above the horizon.
Mars is easy to spot using just your eyes as it is one of brightest objects in the sky (even rivaling Jupiter in brightness) and it has a distinctive red-orange colour. It will becomes easier to see without staying up too late as it rises in the East earlier and earlier: at 8: 50 pm on Sept 13, 6:30 pm on Oct 13th, and 03:07 pm on Nov 13th. In theory the best views come closer to midnight when Mars is at its highest, due South and crossing the meridian. But the face of Mars changes as it rotates so it is said that the best time to view Mars is “all night” to watch different surface features make an appearance.
What You Can See
A telescope is required to see any surface details even when Mars is at its biggest and brightest – Mars’ maximum size in this apparition is just 22.6 arc-seconds – that is small. By comparison, the full moon is more than 80 times larger.
Mars will appear small as shown in this simulated 0.22° field of view and 225X magnification that I obtain with this setup: Skywatcher ED 100 mm f/9 refractor, Vixen 10mm SLV eyepiece & 2.5x Powermate/Barlow.
But even relatively small telescopes (60 to 100 mm) do reveal the major features: the polar caps, lighter areas of rust-coloured dust, and darker areas of exposed volcanic rock. Larger scopes are capable of better resolution and showing more detail.
You’ll want to bump up the magnification, by using a longer focal length telescope or shorter focal length eyepiece, for example. The highest usable magnification depends on the seeing conditions and the aperture of your telescope. Generally, a magnification of 1 or 2 times the aperture in mm works well on nights of good seeing. For example, if you have a 100 mm telescope, try 100X to 200X. If you have a 200 mm scope, try 200X to 400X. However the maximum magnification is usually limited by Earth’s atmosphere as any turbulence will blur the image. Magnifications above 400X may not be realistic no matter how large the telescope.
Simulated magnified view of surface details through a 200 mm (8 inch) Schmidt-Cass telescope. Image credit: generated with CalSKY.
Simple eyepiece designs with fewer glass elements and a narrow field of view can work well. Eyepieces with a shorter focal length will provide a higher magnification. A good 2x or 3x Barlow or Powermate lens can be useful for increasing the image size with your set of eyepieces.
The Changing Face
Mars rotates on its axis at almost the same rate as Earth giving it a day/night cycle that lasts 24 hours, 39 minutes, and 35 seconds. That is good for observing for two reasons. First, you can see different surface features throughout a single night. If you start observing at 08:00 pm on Oct 13th with a dark feature like Syrtis Major located near the western limb then 4 hours later, at midnight, it will have travelled towards the center with the new feature Sinas Meridiani appearing in the west.
The second benefit is that you can observe at the same time on subsequent nights and see a new feature near the eastern limb before it rotates off the face about 40 mins later. The free program Stellarium displays a simulated view of the surface of Mars when you zoom-in enough and you can use it to visualize how features move as Mars rotates.
The changing face of Mars over several weeks near its opposition. Image credit: generated with Stellarium.
With the patience to observe over several weeks, this rotation makes it possible to see the full 360 degrees of the Martian globe.
Prominent Surface Features
The RASC Observer’s Handbook 2020 has a map of the major features on page 221 and there are plenty of others available online.
Image credit: Space Telescope Science Institute, Ann Feild Didyk and Graphics Dept.
Many maps show a view following a convention where “South is up” and remember that your view through your telescope may be inverted (common for reflectors like Dobsonians) or mirror flipped right-to-left (for refractor, compound, or Schmidt-Cass scopes). So check your orientation when identifying features.
The major features of the Martian surface (excluding the polar caps) following the “south is up” convention. Image Credit: Damian Peach/Sky and Telescope.
CalSKY or the online Mars Profiler from Sky & Telescope are useful tools for showing features visible at any observing site and any date/time.
The polar caps are one of the most striking features. The Southern Polar Cap (SPC) is prominent at this time because it is just past the summer solstice in the Martian southern hemisphere. Summer in the south means that the south pole is tilted towards the Sun and, near opposition, also towards us on Earth. But don’t hesitate in having a look for it because the SPC is shrinking and will likely melt away completely during this apparition. It is getting a double whammy of summer heating and additional heating because Mars is at a position in its orbit that brings it relatively close to the Sun. The northern polar cap will not be visible but it is possible to see hazy clouds above the northern region.
Other prominent regions on Mars are differentiated by brightness and colour with lighter areas of rust-colored dust, and darker areas of exposed volcanic rock. The lighter areas were thought to be continents so their names include “land” or “plain” such as Arabia Terra, Hellas Planitia, and Amazonis Planitia. The darker regions were thought to be seas or large patches of vegetation. Examples include Mare Erythraeum, Mare Acidalium and the striking Syrtis Major Planum. These dark regions may appear to change their size and shape over time. Early observers attributed the changes to rainfall or changes in vegetation but it turns out that these regions can be obscured by atmospheric dust or made brighter by the presence of clouds.
If you have a large telescope or the equipment and skill to photograph Mars then you may be able to identify some of the specific features described below.
Dust storms can appear as yellow-ish hazy areas. Local or regional dust storms are an interesting sight but large dust storms can ruin observing by obscuring features – a global dust storm during the last opposition in 2018 covered the entire planet!
Rotating globes from 2018 show a global dust storm completely obscuring the surface of Mars. Image Credit: NASA Hyperwall.
Blueish-white clouds, formed from water ice, may also be visible especially in photographs. Such clouds often appear near the equatorial regions, around the large volcanoes, close to the limb, or close to the northern polar region.
Syrtis Major Planum is one of the darkest regions on Mars. It was observed as early as 1659 by the astronomer Christiaan Huygens and was the first surface feature seen on another planet. It is now known to be a low relief shield volcano but was originally thought to be a shallow sea. The name “Syrtis Major” was chosen by Giovanni Schiaparelli during Mars’ close approach to Earth in 1877.
Olympus mons in the Tharsis Montes region is the largest volcano on Mars, and also the largest known volcanoe in the entire solar system. As a comparison, Olympus Mons is 25 km high and 624 km in diameter with a 80 km caldera at its summit while the largest volcano on Earth, Mauna Loa (10 km high and 120 km wide), is less than ½ the height, ¼ the diameter, and ¼ the height. Volcanoes can grow larger on Mars because of its lower gravity. Also, Mars’ crust remains stationary over a lava hot spot while on Earth crustal plates move above the hot spots spreading the lava among many volcanoes.
Comparison of Olympus Mons to large mountains on Earth. Image Credit: Marspedia.org.
Hellas Planitia is a large impact basin located in the southern hemisphere. Hellas can appear so bright (due to fog, surface ice, and clouds) that it is sometimes confused for the southern polar cap. It is likely to have been formed by an asteroid impact early in Mars’ history – about 4 billion years ago.
Valles Marineris is a large system of canyons that runs along the equator of Mars. It is the largest canyon system on Mars and is almost 5 times deeper than the Grand Canyon. The large canyon system was discovered in 1972 by NASA’s Mariner 9 spacecraft, the first satellite to orbit another planet.
Solis Lacus is also known as the “The Eye of Mars” because it is a dark circular feature surrounded by a light area, as is a pupil. Solis Lacus is known for the variability of its appearance, changing its size and shape when dust storms occur. Percival Lowell believed that it was the planetary capital of Mars due to the number of “canals” he claimed intersected at the region.
Surface features are apparent in this high Resolution image of Mars from September 11, 2020 taken with a Mewlon 250 telescope. South is up so the bright white patch at the top is the Southern Polar Cap. The large uniform light area at the right in the north is Amazonis Planitia. Clouds are visible around the Arsia Mons volcano in the Tharsis Montes region. More hazy bluish-white clouds appear on the left limb and above the Northern Polar region. Solis Lacus is faint just below center near the left limb. Image Credit: Damian Peach www.damianpeach.com.
Schiaparelli Crater is a large impact crater measuring approximately 460 km in diameter. It was named after Giovanni Schiaparelli, an Italian astronomer known for his observations of the Red Planet and his mistranslated term “canali”. In the book and movie, The Martian, Mark Watney, a stranded astronaut from the Ares 3 mission (the 3rd manned mission to Mars) makes a 3,000 km trek from Acidadia Plantia to Schiaparelli Crater to reach the landing site of Ares 4.
FInal Observing Tips
Mars only gives us a small view and it can be difficult to pick out the even tinier features on its surface, so here are some final tips:
Have patience. Observing is a learned skill that takes practice.
Pick a Night with steady air. Details are easier to spot when the air is steady and the stars aren’t twinkling too much.
Acclimatize Your Telescope. Bring your scope outside to acclimatize for at least 30-60 minutes before you plan to observe. This will help reduce the air currents inside your scope that degrade the image. Scopes with large mirrors or lenses, and those with closed tubes, take longer to acclimate.
Observe Frequently. Take advantage of the fact that Mars rotates slower than the Earth by extending your observing session for several hours on one night, or at the same time over several days to see all sides of the planet.
Relax and sit down. An old rule of thumb says that observing while comfortably seated is the equivalent of adding a couple extra inches of aperture. When you observe seated you are more relaxed and less shaky, and that pays off in terms of being able to see more detail.
Mars is one of the most interesting and rewarding objects in the solar system to observe and next few weeks provide the best opportunity in the next 15 years to view it.
The upcoming opposition of Mars promises to be an exciting event for planetary observers – here is the Who, What, When, Where, and Why on the 2020 opposition.
Hubble’s Mars image indicating major features on the face of the planet. Image Credits: NASA, ESA, the Hubble Heritage Team (STScI/AURA), J. Bell (ASU), and M. Wolff (Space Science Institute)
Who: The planet Mars and you – to observe it! Mars is the 4th planet from the Sun and is named after the Roman god of war. It is also called the red planet because of its rusty red surface. It is the second smallest planet with a diameter that is about ½ that of Earth’s and whose gravity is only 37.5% of Earth’s. A big attraction of Mars as an observing target is that it is the only planet to reveal its surface features to us with backyard telescopes with oppositions being the best times for a chance to view those features.
What: Oppositions occur when the Earth passes directly between an outer planet and the Sun placing the planet opposite the Sun in our sky – the planet rises when the Sun sets and it can be viewed throughout the entire night. An added bonus is that a planet at opposition is close to Earth and therefore appears bigger and brighter.
The dramatic change in the size of Mars around its 2020 opposition. Image Credit: ALPO (Association of Lunar and Planetary Observers)
Mars displays the greatest changes in size because it is the first outer planet away from the Sun from us. It can go from being a fairly small and faint dot to the 2nd brightest planet in the sky (after Venus). Surface features like the polar ice caps, volcanoes, and darker regions of exposed volcanic rock become visible at opposition. A Martian sol lasts slightly longer than an Earth day so new surface features appear night after night and you get a chance to see much of Mars’ surface in the weeks surrounding opposition. Mars appears small, even at its maximum size, so a telescope is required to see surface features. Bumping up the magnification and some patience can help in picking out the details.
When: Opposition occurs at 23:20 UT on October 13, 2020. That is 04:20 pm Pacific Daylight Time but you don’t need to aim for the exact date or time – views of Mars will be good for several weeks around opposition. In fact, Mars makes its closest approach to Earth on Oct 6, 2020, a little bit earlier than the opposition date due to the elliptical (non-circular) shape of its orbit.
The minimum distance to Earth, in astronomical units (AU), and maximum disc size, in arc seconds (“), for some oppositions. Image Credit: SkyNews Magazine.
Oppositions of Mars occur on average every 780 days or approximately every 26 months. The distance from Earth to Mars varies between oppositions as does its size. Mars will be about 0.41 AU from Earth at this year’s 2020 opposition with a size of 22.6 arc-seconds. The 2020 opposition ranks high with respect to distance and size as Mars will not be as close nor as big during the next three oppositions in 2022, 2025, and 2027.
Where: Mars will be in a good position for observers in the Northern Hemisphere during its 2020 opposition. Mars will rise in the east at sundown (06:30 pm PDT) and will climb higher into south-eastern and southern skies closer to midnight. It reaches a maximum altitude at opposition of 46 degrees above the horizon at 1:00 am PDT on Oct 14th – there is likely to be better seeing and less atmospheric distortion with Mars that high in the sky.
Stellarium chart showing the Location of Mars at 10:00 pm on Oct 13, 2020 from Vancouver, BC.
Why: Oppositions of Mars occur because the orbits of Mars and the Earth make them align in a straight line with the Sun where the Sun and Mars are on opposite sides of the Earth.
Oppositions occur when an outer planet is lined up with the Sun and the Earth. Image Credit: Marsopedia
The Earth moves more quickly in its orbit than Mars so it passes and then catches back up to Mars every 780 days on average.
That’s it. Go out and see Mars for yourself. Try to observe it over a few nights around the opposition to take in more of its surface. Keep an eye on this website for additional upcoming articles on Mars. With some luck with the weather and clear skies, Mars will reveal its surface details to us Earthbound observers.
A few years ago I wanted to have a look at the Triangulum Galaxy M33 from my yard in Coquitlam. There’s a significant amount of light pollution – I’d estimate the sky to be Bortle class 7 to 8. Despite that, I am able to see bright Messier objects like the Globular Cluster M13, the Ring Nebula M57, and the Bode’s Galaxy M81. M33 was more challenging. I tried several different nights but had no success in spotting it. Confused and frustrated, I checked the magnitude in Stellarium as I knew magnitude is a measure of an object’s brightness. Stellarium showed M33’s magnitude as 5.7 – quite bright and definitely brighter than the other objects I could see: M13 at 5.8, M57 at 8.8, and M81 at 6.94 (remember that brighter objects have a lower magnitude). So why couldn’t I pick out M33?
I have since learned that magnitude works well for stars but it is not a good indicator of how easy or hard it is to see deep sky objects that cover an extended area – aka extended objects. Magnitude assumes all the light is concentrated in a single point source like a star but M33 covers an area of approximately 65 x 40 arc minutes and its light is spread over this entire area.
Simulated average vs total brightness – on the left, the light is spread over a faint 2×2 square while on the right, the light is spread over a 1×1 square. The total brightness for both squares is the same but the average brightness is much less for the square on the left because its area is 4 times larger.
A better measure is “surface brightness”: the average brightness over the extended area. Surface brightness is often measured in magnitudes per square arcsecond. M33 has a surface brightness of 23 mag/arcsec^2. If you divide the area of M33 into a bunch of little 1 arcsecond squares then on average each square will be as bright as a 23rd magnitude star. As with magnitudes, a lower surface brightness indicates a brighter object. An object with a surface brightness greater than 22 is generally considered to be faint.
Surface brightness is also a good way to measure sky-glow and light pollution. An urban/city sky might have a surface brightness of 17 mag/arcsec^2 while a pristine dark sky might have a surface brightness of 22 mag/arcsec^2. The Canadian company Unihedron makes relatively inexpensive Sky Quality Meters (SQM) used by many amateur astronomers to measure sky-glow. I used Unihedron’s SQM-L and measured the sky-glow at my Coquitlam location to be 18.5 mag/arcsec^2 on a moonless night.
An object can be difficult to detect when its surface brightness is close to or fainter than the sky-glow. Tony Flanders reports barely seeing objects down to a surface brightness of 21 in his skies where the sky-glow is 18 mag/arcsec^2 and he speculates that an object is not detectable if it is more than 3 magnitudes fainter than the sky-glow. With this in mind, it is not surprising that M33 is hard to detect from my Coquitlam location as the surface brightness of M33 is almost 4.5 magnitudes fainter than the sky-glow.
There are other factors involved in whether or not an object is visible. The RASC Oberver’s Handbook section “Magnification and Contrast in Deep-sky Objects” has a good explanation of some of these including the following.
Surface brightness is not uniform. For example spiral galaxies often have a bright core area that is brighter and easier to see than the arms. Flanders and others suggests that peak surface brightness is an even better indicator of whether or not an extended object is difficult to observe.
Larger objects are easier to detect so increasing the magnification on faint objects can help.
Objects like nebula are easier to see when using band filters that increase contrast by darkening the sky-glow more than they darken the light from the nebula.
Surface brightness may not be a good measure for objects like globular and open clusters that resolve into individual stars.
The surface brightness of an object is subjective as it depends on the size of the object. How far do the faint arms of a spiral galaxy really extend?
Enable the “Additional Information” in Stellarium’s Configuration to display the selected object’s surface brightness.
Several tools can help you find the surface brightness for observing targets:
The surface brightness of an object can be displayed in Stellarium if the “Additional information” option in the “Configuration” window is enabled.
The “Magnitude and Contrast Calculator” is a spreadsheet, included as a supplement to the RASC Observer’s handbook, that calculates the surface brightness and predicts whether an object is visible from telescope, filter, and sky parameters.
Tony Flanders has compiled a list of Messier objects that includes the surface brightness and peak surface brightness (for some Messier objects). The table below is an excerpt from his list.
