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Thread: Bad Science analogy: Stars and Cars

  1. #1
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    Bad Science analogy: Stars and Cars

    Last night a professor of astronomy at one of our local universities was hosting an astronomy club night at the university's observatory. He mentioned the prodigious fuel consumption rate and short lifetime of massive stars and likened it to the faster burning of fuel by heavy motor vehicles as compared with lighter ones. In my opinion it was a poor analogy because it did not take into account what the vehicles can deliver as they consume their fuel. My four-seat car can make about 25 miles per gallon (mpg) in long-haul highway driving, which amounts to 100 seat-miles per gallon (smpg). A 40-seat bus with ten times its weight can make about 5 mpg, which is 200 smpg. The bus also has about ten times the fuel capacity, so it can go about twice as far between refuelings. In terms of running time as an analogy to a star's lifetime, the bus can run for a longer time on a full load of fuel, a far cry from the situation with the higher-mass stars.

    I had whom I considered a reliable source about the bus, specifically the driver on a charter trip I once rode. He kept a detailed log, as required by law, of miles driven and fuel purchased.

    I once wrote a letter to Scientific American about this issue, when the author called high-mass stars the SUVs of astronomy. He got snarky with me, so I dropped my subscription.
    Last edited by Hornblower; 2019-Jul-15 at 10:24 PM. Reason: Fix syntax error

  2. #2
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    It seems like a kind of obvious analogy to make, since everybody knows about big cars, but I understand your point.

    Another issue actually is that it doesn't really do justice to the scale. It's true that SUVs are bigger than smaller passenger cars, but the size difference is nothing like the difference between the sun and really massive stars, as illustrated for example in this video.
    As above, so below

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    It seems logical to offer some analogy regarding fuel consumption since star fusion is an energy conversion process, and chemical burning is something we understand in a visceral way. Often what can be lost in technical quality is overcome by how relate-able the analogy is to everyday life, especially if it is serving as a simple starting point for something far more complex.

    I think boat motors would make a better analogy. My parents had a boat with a fuel rate based on the number of gallons per mile.
    We know time flies, we just can't see its wings.

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    My fundamental point is that attempts at analogies with motor vehicles or machines of any type do not address the rapid burnout of high-mass stars. The stars at their moment of formation are fundamentally alike except for their size. In my opinion having the Sun correspond to a car and a high-mass star to a boat does not cut it. To a first approximation a car and a bus, both with internal combustion piston engines, consume fuel at rates proportional to their masses. That is just the power needed to move the respective masses. The car and the bus are fundamentally alike except for their difference in mass. With stars, the physical properties of hydrogen are such that the more massive star consumes its fuel disproportionately faster. My advice to the professor would be to just touch on that point and don't bother with analogies to machines where those properties do not apply.

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    Quote Originally Posted by Hornblower View Post
    The car and the bus are fundamentally alike except for their difference in mass. With stars, the physical properties of hydrogen are such that the more massive star consumes its fuel disproportionately faster.
    That's why I think boats would be better. The fuel consumption rate isn't linear with boat mass due to the exponential increase in water resistance. Wind resistance is also exponential but it is trivial for the common car and bus driver. The water "pressure" increases exponentially with boat size, so the fuel consumption (gpm, not mpg) also increases exponentially, depending on boat length and some other variables, but it isn't meant to be a close analogy.

    Perhaps a better analogy would be the energy necessary to lift and carry steel balls that vary in diameter, given the cube law to obtain mass.
    We know time flies, we just can't see its wings.

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    I am discussing fuel burn rate as a function of mass, not linear measurements.

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    As I said earlier, I agree that it's not a good analogy. If you really want to make an analogy, maybe a nuclear weapon versus a nuclear power plant, but even that is tricky because it's not that the bomb is bigger, but rather it's the specific configuration and fuel mix.

    Another thing also, in addition to what I wrote about the enormous difference in mass between the sun and large stars, it's also a chance to talk about how incredibly low in power the sun actually is (apparently, it is like a compose pile in terms of the energy it puts out). So I think there are lots of really interesting things about stars that you don't get from analogies like cars or boats.

    For a headline title, "The SUVs of the galaxy" might be OK, but I wouldn't go further than that.
    As above, so below

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    Once again, these attempts at analogies do not address the property of stars, which is that the burn rate of the more massive star is enormously disproportionate to its mass, even though they are fundamentally alike except for the difference in mass. A main sequence star of 10 solar masses has a bolometric luminosity of roughly 10,000 times that of the Sun, which means it burns out in about 1/1,000 as much time. No heavier but otherwise similar motor vehicles, ships or aircraft do anything of the sort in normal operation. As for SUVs, I don't think they are worse gas guzzlers than other styles of motor vehicles of the same weight and engine type.

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    My feeble brain just now recalled a good analogy, one which was discussed at length in a previous thread last year or perhaps earlier. Let the Sun be analogous to a small, well insulated house with the windows closed on a cold day, and a high mass star analogous to a big, poorly insulated house with the windows open. We will need disproportionately more fuel or electricity to keep the big house warm inside. The insulation is the key to making an analogy with the stars. The stars' internal dynamics are such that the more massive one reaches equilibrium in a more bloated, low density state. This makes it more translucent, so the radiant heat escapes faster. As a result it contracts more than it would have with greater opacity, gets hotter inside in spite of the heat loss, and induces enough more fusion to stabilize itself while still more rarified than the Sun.

    In my opinion this would have been a good choice for the professor at the observatory, because we the people at large can understand household heating bills and the insulation or lack thereof that affects them. We can understand that just as well as fuel consumption in our cars.

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