The Astronomy Nexus

I've got some pesky spammers here, doing the old "stick a bunch of spam in older posts" stunt, and since it's been pretty quiet the last couple of months, I've turned off commenting for the near future. It'll probably come back after a general site upgrade in the next few weeks...

I have updated the HYG Database. It now includes the complete Hipparcos catalog (the old version had only stars to magnitude +9.0), and it includes velocity information where it's available.

More details are on the main HYG Database page linked above.

April in the northern hemisphere means galaxies -- lots of 'em. But what do you do if you live in the city, can't get to a dark-sky site easily, and it's close to full moon?

How about a few stars, for a change? Recently, I took a look at several bright double stars, for a change of pace from my usual choices of planets and deep-sky objects. More details below.

I have made another significant update to the Distant Worlds Star Mapper. It now draws much more realistic star images. There are some sample screen shots after the break:

In the various discussions about the dynamics of the potentially habitable planet around Gliese 581, the issue of tide-locking comes up. How do we know that a planet like this may face this problem? It's closer to the parent star, of course, but the star is a lot less massive as well.

It turns out it's not too hard to estimate the tidal forces, at least at the order-of-magnitude level, for a clone of Earth put around another star. More details below.

Gliese 581, a red dwarf star already known to have at least one other planet, has a potentially habitable planet. It's somewhat larger than Earth (about 5 times as massive), and its average temperature appears to be well within the liquid range for water.

First off, this detection is amazing. Just finding something like this is a challenge; most exoplanets are much more massive, making them that much easier to detect. And then there's the question on everyone's mind: Is this planet truly Earthlike? It's reasonably sized, and could have the right temperature, but beyond that, it's really hard to answer. For one thing, it already has a massive planet, about the mass of Neptune, orbiting even closer still. This complicates the system's dynamics significantly. The smaller planet's orbit is obviously stable in the long term, but even small orbit changes can have significant effects on a planet's climate. As far as I can tell, this is still an open question; astronomers haven't really figured out whether or not a large planet like this would make the smaller, more Earthlike planet uninhabitable. Additionally, red dwarf stars can subject planets to unusually high tides compared to larger stars. In extreme cases, this can lead to a planet having one side always face its star, much as the Moon always keeps one face towards Earth.

UPDATE: The overall dynamics may not be a severe problem. Steinn Sigurdsson writes:

In short, the planet configuration is stable and the inner Neptune mass should have neglibible influence on the middle planet (other than being a spectacular evening/morning star) - only way it would affect the climate on the middle planet is if it affected tidal locking process - the middle planet is just outside a 2:1 orbital resonance with the inner planet - hm, in fact it might be in the 12:5 resonance which is a bit weird - but their final orbital configuration might have been dynamically coupled, this could influence whether the middle planet locked to its orbital period in 1:1 rotation resonance.

Of course, we still don't know enough about the atmosphere, actual temperature, etc. to answer other questions about habitability.

If there were intelligent beings living here, what would they experience? More below.

I've made a few significant changes to the Distant Worlds Star Mapper. There are two new features available:

1. Tooltips for star information
2. Viewing star charts at different times (the far past and future)

Here's a sample star chart showing both features: this is what the sky from Barnard's Star will look like in 12,000 years. A certain well-known star is identified in the center of the view:

The Sun from Barnard's Star in 14,000 AD

I've created some more examples, and a description of the new features, below the break.

I haven't posted a whole lot of updates in the last year. That's because I've been busy with something else huge: adopting a beautiful 6-year old girl from China.

As you can imagine, this has kept me pretty busy, so this site's been pretty quiet. More pictures below...

You've probably heard the news a few days ago that astronomers came up with a new definition for "planets" that added 3 new planets to the solar system (including the largest asteroid, Ceres). A couple days ago, another definition was proposed, which appears to be more consistent in some ways. However, it excludes the farthest traditional planet, Pluto. By and large, professional astronomers don't care too much one way or the other -- reactions like this (Rob Knop) and this (Phil Plait) are pretty typical. In general, what a "planet" is matters more to the general public than to the pros.

Even given that, you do have to wonder what the fuss is about. It's pretty clear nowadays that Pluto is a very small object, different from the other 8 traditional planets. In fact, Pluto is more like the largest non-planetary bodies in some ways. Pluto is about a dozen times more massive than the largest asteroid (Ceres), but the smallest remaining planet, Mercury, is more than 20 times more massive than Pluto. Seven satellites (our Moon, the four large moons of Jupiter, Saturn's moon Titan, and Neptune's moon Triton) have more mass than Pluto. If Pluto had been discovered yesterday, or even 20 years ago, rather than almost a century ago, it almost certainly would not have been called a planet at all!

So why was Pluto ever considered a planet in the first place, rather than something like a really large, icy asteroid? It turns out there is a lot of interesting history behind this, involving a couple of great planet hunts in past centuries. More below.

If you're a Northern Hemisphere observer, you'll notice that at this time of year, the Milky Way isn't visible. In fact, during late-spring evenings the Milky Way roughly circles the horizon -- almost impossible to see even on a dark night -- which means the sky overhead, or close to it, is as far from the Milky Way as possible.

You may remember that the Milky Way Galaxy roughly resembles a flattened disk, much thinner than it is broad. Like most stars, the Sun stays within this disk. When you look in or near the plane of the disk, you see enormous numbers of stars, but when you look straight out of the disk -- towards the "galactic poles" -- the sky looks relatively empty.

The view there may be empty of nearby stars and nebulas, but not of objects outside the Milky Way. With fewer stars, nebulas, and other galactic material to interfere, external galaxies are much easier to see. By a curious coincidence, a very rich collection of galaxies -- the Virgo Cluster, named after the constellation most of them appear in -- exists right near the north galactic pole. More below...