Summer only lasts so long. Like everything, it’s transient. That will be our theme as we explore wispy noctilucent clouds, a nova that can’t sit still, and a supernova in NGC 5427 in Virgo.

Noctilucent clouds near Duluth, Minnesota
Electro-blue noctilucent clouds reflect in a small lake near Duluth, Minnesota, on June 27, 2021, around 10:30 p.m. The extent and brightness of these exotic clouds have been increasing in recent years. Once confined to high latitudes, they've recently been seen as far south as southern California.
Bob King

Since mid-June I've been on a vigil. Every clear night at 9:55 p.m. I drive to a nearby location with an unobstructed view of the northern sky hoping to see noctilucent clouds. Half terrestrial and half celestial, these night-shining clouds form around 80 kilometers up in the mesosphere, far above the feathery cirrus and cauliflower cumulus of a summer's day.

Lower clouds literally appear out of thin air when water vapor condenses on specks of dirt, salt, and industrial pollutants. Noctilucent clouds (NLCs) instead use soot shed by incoming meteoroids and, in some cases, sulfur-rich volcanic gases for that purpose.

blue noctilucent clouds hover over trees and a pink sunset
Resembling a sprawling nebula viewed through a telescope, noctilucent clouds blanket the lower third of the west-northwest sky an hour after sunset on June 27, 2021.
Bob King

They also glow blue, at times intensely so. Sunlight reflecting off the clouds must first pass through the ozone layer on its way to our eyes. Ozone absorbs red light and lets blue through. Finally, because of their great altitude NLCs are visible long after sunset, when other clouds are steeped in shadow.

My watch begins an hour after sunset. There's no way to know in advance if they'll show. More often than not, I don't see them. But if they decide to appear, the first wispy tendrils materialize very low in the northern sky around 10 p.m. local time.

While the clouds can persist for more than two hours after sunset in the northern U.S., the peak viewing time occurs around 70–85 minutes after sundown (or before sunup), when they contrast best against the darkening sky. Use this calculator to find your local sunset and sunrise times.

this chart shows where noctilucent clouds form in the mesosphere portion of the atmosphere
Familiar clouds form when water vapor condenses on dust, salt, and other minute particles in the troposphere, the lowest atmospheric layer. NLCs, also called polar mesospheric clouds, appear when water vapor from the lower atmosphere crystallizes on meteor "smoke" near the top of the mesosphere.

NLCs rarely climb higher than 5°–10° above the horizon from my 47° north latitude. But they're not always so restricted. On June 27th, while driving to my cloud observatory (a road pullout), a surprise batch of clouds rose up in the western sky. I couldn't believe my bad luck. On reaching the clearing I was shocked to discover it was a massive display of NLCs!

Looking like those stretchable spider webs used as Halloween decorations, strands and whorls of blue cloud extended from just south of west clear around to the northeast and up to 30° in altitude. After taking notes and photos I drove to a nearby lake to capture their reflection in the company of fireflies.

Wet and cold
Temperatures near the top of the mesosphere typically hover around –90°C (–130°F) but drop to their lowest around the time of the summer solstice. Data from the MLS on the Aura satellite show a steady decrease in temperature from May 1 to June 3, 2020 (left) — down to -128°C (–198°F, note the original scale is in Kelvin) — and a concurrent increase in water content (right). Increasing water levels promote NLC formation.

NLCs require cold air to form. Strange as it sounds, air in the mesosphere ascends and gets colder in the summer and descends and warms in the winter. According to Cora Randall, the principal investigator for the Aeronomy of Ice in the Mesosphere - Cloud Imaging and Particle Size Instrument (AIM-CIPS) at the University of Colorado-Boulder, NASA’s Microwave Limb Sounder (MLS) data indicate that high-altitude temperatures are lower than average and water vapor concentrations higher than average in the 2021 season, ideal conditions for NLC formation.

"If viewing conditions are favorable on the ground, I would not be surprised to see more reports in the next week of noctilucent clouds down to 40° north or even a bit lower,” Randall said in a June 29, 2021, Earth Observatory post. 

Time-lapse of noctilucent clouds over Alberta on July 7, 2020.
Chris Ratzlaff

NLC season runs from mid-May to mid-August in the Northern Hemisphere and from mid-November through mid-February in the Southern Hemisphere. The clouds appear at both dusk and dawn.

Recent research suggests that human-caused climate change has increased the frequency of the clouds and also pushed them to lower latitudes. A large outbreak earlier this month brought them into view as far south as Valencia, Spain, at latitude 39° north. In June 2020, NLCs pushed the limit even further, making a surprise appearance at Joshua Tree, California, at latitude 34°.

I've seen six displays of NLCs from northern Minnesota so far this summer, the most I've ever seen in a single season.

Noctilucent clouds form over a lake with fireflies
By 11 p.m. local time on June 27th, nearly two hours after sunset, wavy NLCs still glowed near the northern horizon. Also visible were red-colored NLCs (left). I suspect that the Sun was so far below the horizon that its light deeply reddened the clouds, masking their normal blue color. The green wiggles are fireflies.
Bob King

Methane emissions from industrial activity appear to be behind their increasing frequency and brightness. In the late 1800s NLCs were a rare sight reserved for observers living in near-Arctic regions. Now in the 2020s, attentive middle-latitude skywatchers can often witness several displays each season.

A satellite image of the north pole
NLCs mostly congregate over the polar regions. NASA's AIM satellite took a series of images on June 26th centered on the North Pole that were stitched together to create this view. Hudson Bay is arrowed for reference.

"When methane makes its way into the upper atmosphere, it is oxidized by a complex series of reactions to form water vapor. This extra water vapor is then available to grow ice crystals for NLCs," said James Russell of Hampton University, principal investigator of NASA's AIM mission, in a press release.

Randall thinks other factors may also contribute to more favorable NLC conditions, including stronger atmospheric circulation with "more ascending air producing more cooling and an increase in water vapor." At the same time, the current weak sunspot cycle produces less ultraviolet (UV) light to break up water molecules at high altitudes."

Keep a lookout for NLCs. Their eerie color and wispy, fibrous shapes possess an otherworldly beauty, while their lately appearance outside their traditional boundaries may be a bellwether of climate change.

Nova Cas still swingin'

Nova Cassiopeiae (V1405 Cas), discovered on March 18th, rose to magnitude 5.1 in early May, when it was faintly visible without optical aid. Later that month it faded to magnitude 8 and has since undergone at least three episodes of dimming and re-brightening. On June 28th, during its most recent upswing, the nova climbed back up to magnitude 6.5, and then quickly dove to 7.5 two nights later.

a star chart of Cassiopeia, with the slow nova V1405 Cas shown
Use this map to find the nova in binoculars. Stars are shown to magnitude 9. The stars marked with the magnitudes are also plotted on the AAVSO map below.

This lively behavior makes a great pleasure to watch, and you don't even need a telescope. It's also up at a convenient hour, appearing in the northeastern sky as soon it gets dark.

V1405 Cas is classified as a slow nova, one that dims two magnitudes in 80 days or more, compared to a fast nova which experiences the same change in fewer than 25 days. A good example of the latter is the recent Nova Herculis 2021 (V1674 Her), which shot up to 6th magnitude on June 12th, then nosedived to its current 12.8.

the light curve of V1405 Cas
The light curve of V1405 Cas from mid-March through late June 2021 reflects its bumpy evolution after the nova eruption. Dates are along the bottom and a magnitude scale at left.
Courtesy of the AAVSO

Fluctuations in a nova's brightness are caused by changes in its spectrum that depend on which atoms are emitting light in the expanding cloud of ejecta during the heat-up and cool-down. Longtime American Association of Variable Star Observers (AAVSO) observer Sebastián Otero of Buenos Aires, Argentina, shared more details of the process in an email exchange:

"It seems that the interaction of the nova ejecta with the interstellar medium, with the subsequent changes in physical conditions, such as the density, recombination and ionization rate, visible when looking at the spectroscopic evolution, is then responsible for the light curve changes."

Wide-field chart for V1405 Cas
AAVSO wide-field chart for V1405 Cas.
Courtesy of the AAVSO

Since Nova Cas will likely remain bright for months, I encourage you to make observations of its changing brightness and submit them to the American Association of Variable Stars. Use the chart here and note the time to the minute of your observation. Then register for a free account at the AAVSO (you can upgrade to become a member later if you wish).

Once you're logged in, go to your account page and under Personal Information, request an observer code. Next, on the homepage, scroll up and click on Tools and Observer Resources, then WebObs and Submit — Individual Observations. It's that easy, and you'll also be making a contribution to science.

Got an 8-inch? Check out this supernova

two galaxies, NGC 5427 and NGC 5426 shown interacting as 2021pfs is visible
Robert Spellman, who worked on the staff of Griffith Observatory for 17 years, took this photo of the supernova and Arp 171 pair on June 27, 2021. At the time, the magnitude of 2021pfs was 14.0. North is up.
Robert Spellman

What does a white dwarf star look like when it blows up? Shimmy over to NGC 5427, a 13th-magnitude spiral galaxy in Virgo, for a look at the recent supernova 2021pfs. Discovered by the automated Zwicky Transient Facility (ZTF) on June 9th, this Type Ia supernova now shines at 14th magnitude 36.7″ west and 2.9″ south of the galaxy's center.

A star chart of Virgo that shows the location of Arp 171
Supernova 2021pfs is located in NGC 5427 in Virgo 3.1° west of Iota (ι) Virginis. View it best as soon as the sky gets dark. Stars are shown to magnitude 9.

NGC 5427 forms an interacting pair with the spiral galaxy NGC 5426 to its south. Together they're known as Arp 171 and located about 130 million light-years away with a combined diameter of some 130,000 light-years. In a faraway time, they may collide and merge the same way the Milky Way and Andromeda spirals are expected to 5 billion years from now. As you tease the supernova into view, you might easily imagine them as a glimpse into the future.


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