A week ago I only heard them, but on Wednesday I finally saw my first robins. Many of us look forward to the change of seasons and anticipate signs of their arrival.
Seasonal changes usually start slowly – a bare spot of ground opens on an otherwise snowy road or we hear the first toot-toots of a saw-whet owl at night – but they can reach a tipping point in a surprising hurry. Today, robins are everywhere and our yard is nearly free of snow.
In a similar way, we anticipate seeing certain stars in the nighttime sky. Stars that are associated with the seasons. I thoroughly enjoy watching Orion climb the eastern sky on November nights, reminding me that winter will soon be getting down to business. Mid-April nights see the decline of Orion in the southwest and the introduction of new stars in the eastern sky. We’ve met a few already – Arcturus, Spica and one non-star celebrity, the planet Saturn.
Let me introduce you to Vega, a star that walks the line between spring and summer. Like the robins, you have to hunt a little to see it now, but before you know it, it’ll be right in your face.
Start with our dependable friend, the Big Dipper, and follow the arc of its handle southward to the first bright star you see. That’s Arcturus. Next, shoot a line from Arcturus about four fists in length straight toward the northeast horizon. If your view is unobstructed, you can’t miss Vega. It’s the brightest, twinkliest star in the region.
Vega is dimmer than Arcturus by a hair, but the most noticeable difference between them is color. Arcturus is an orange giant with a cooler surface temperature (7,250 degrees F) compared to Vega’s 16,650 degrees. Whiter means hotter and redder means cooler when it comes to stars.
Vega is similar to the sun in star type, being a member of what astronomers call the main sequence. These stable stars ‘burn’ hydrogen in their blazingly-hot cores to create energy, starshine and leftover helium ‘ash’. Most of the stars in the sky belong to the main sequence.
As stars age, they leave the main sequence to become giant and supergiant stars that burn through additional elements to create energy. Arcturus burns its helium ash into carbon and oxygen. Changing fuel type changes a star’s equilibrium, causing it to expand into a stellar giant. Main sequence stars that were once sun-like in size balloon out, cool and redden as they age.
Both the sun and Vega will switch over to helium burning in the distant future. As they do, astronomers expect them to evolve into big orange puffballs much like Arcturus. Will the sun grow large enough to fry the Earth to a crisp? Do you really want to know? The answer: most likely. Not to get too concerned yet. That unhappy day is still almost five billion years in the future.