I think anybody can run a Stellar validator node[1], somewhat akin to anybody being able to set up a full Bitcoin node.
Ripple is completely centralized in all meanings of the word, and a lot of people confuse the two since Jed McCaleb is a shared founder between the two projects[2]. Having said this, the Ripple Foundation has tried to dispel this notion and claims that they have a roadmap for the Ripple ledger to become more decentralized[3], so perhaps it's fairer to say that Ripple is centralized today but in the future plans to become less so as they try to onboard 3rd party validators.
IBM seems to have stood up a large proportion of verified Stellar validator nodes[4].
That information may not be accurate, but either way there's confusion between two related things: Stellar and lumens.
Stellar is the technology which anyone can use to create tokens which may or not be exchangeable for other types of tokens created with Stellar. You can control 100% of the supply of the currency type of your choice within Stellar.
The lumen is the native asset of the Stellar network intended to stabilize the decentralized network. This is mostly pre-mined. Its intended use is as a bridge to facilitate exchanges between different currency types. An inconsequential amount of lumens are used as transaction fees (less than one hundred-thousandth of $0.01) to fight spam.
I think the Stellar Foundation distributed 20% of the tokens so far (from the dashboard[1]), and holds back 5% of available token supply for their operating costs.
Your point still stands though: like Ripple, Stellar's token supply is still in early stages of distribution which gives the respective foundations immense influence over the nascent networks.
Does this mean that Stellar is completely decentralized, or is there some component that is still centralized? I'm not very knowledgeable about Stellar.
Most of the outstanding token supply is still held by the Stellar Foundation to be distributed. In my books, that means that it's still pretty centralized, but much less so than other ICOs or early coins that seem like outright scams.
The most charitable interpretation is that Stellar is on the right path to decentralization.
"Although there have been many attempts to corner markets by massive purchases in everything from tin to cattle, to date very few of these attempts have ever succeeded; instead, most of these attempted corners have tended to break themselves spontaneously."
Whether a cryptocurrency is centralized or decentralized has a specific technical meaning, namely whether a party is, by design, in control of the security or trust of the currency and can unilaterally make decisions. By that definition, Bitcoin is decentralized and Ripple is not.
If you remove the current state of the token supply, then I'd say that Stellar seems to be relatively decentralized as evidenced by the number of verified validators not under direct control or ownership by the Stellar Development Foundation (around 13% by number; don't know the validator splits by volume).
Ripple is completely centralized in all meanings of the word, and a lot of people confuse the two since Jed McCaleb is a shared founder between the two projects[2]. Having said this, the Ripple Foundation has tried to dispel this notion and claims that they have a roadmap for the Ripple ledger to become more decentralized[3], so perhaps it's fairer to say that Ripple is centralized today but in the future plans to become less so as they try to onboard 3rd party validators.
IBM seems to have stood up a large proportion of verified Stellar validator nodes[4].
[1] https://www.stellar.org/developers/stellar-core/software/adm...
[2] https://en.wikipedia.org/wiki/Jed_McCaleb
[3] https://ripple.com/dev-blog/decentralization-strategy-update...
[4] https://dashboard.stellar.org/